Traditional coal ash sampling and analysis using a laboratory facility can take a few hours if there is an on site lab or days if the samples are sent away for analysis. Using the Parker Bretby Gammatech portable Ash Probe, the results are available in a few minutes by probing the coal pile with no special training required by the operator. All the data is collected during the shift and can be downloaded onto a memory stick or direct USB cable to a PC in CSV format for analysis and reporting. The unit comes with the Parker Bretby Gammatech utility software for ease of download. This white paper describes the journey the Parker team made in producing the new AshGraffix controller for the Ash Probe.
During the latter part of 2012, we were advised by our supplier that the micro processor used in the Ash Probe system was being made obsolete; this gave us two choices:
1. Incorporate the new processor and modify the software for the existing display unit.
2. Develop a new design display unit based on our customers feedback using the latest processor.
Fig 1. Old QWERTY keyboard display unit with LCD display in English only.
To support sales of the Ash Probe while we developed the completely new unit, we chose to write the new software code for the existing display unit as the old processors were getting harder to obtain; this was essential so we did not leave customers waiting for the upgraded version of the display unit and gave us the opportunity to upgrade older units coming back for repair.
Our customers gave us some important clues as to what features they would find useful in a new unit:
During 2014 we set out to design the new product and software called AshGraffix with the customer needs in mind. The project time frame was 12 months from concept so we would be able to demonstrate the first unit at mining shows in 2015. The design for hardware, electronics and software was all done in house by the Parker team. Support from our local suppliers was critical as we went through several iterations of the printed circuit board design as this was the first time we had used surface mount components and touch screens. Finding low power components was key to the end result as the unit needed to work a full shift in some very harsh conditions from the heat and humidity of India and Vietnam to the low temperatures and dry conditions in Mongolia and Siberia.Component list
Neil Jenkinson, our mechanical Engineer, selected an aluminium extrusion for the outer case that was rugged and could hold the printed circuit board and touchscreen. He found a superb membrane protector product that would sit on top of the touchscreen preventing scratching and potential failure if sharp items hit the screen by accident or in duality use, e.g a pen tip to tap the screen. The industrial cable connectors were retained as they had given good service over 20 years and readily available for after market sales. Different types of of battery packs were tested for durability and full function duration with industry standard Metal Halide rechargeable cells coupled with an intelligent charger selected as the best all round option for the AshGraffix.
As Human Machine Interfaces (HMI) have become more prevalent in the industrial world and availability for colour touchscreens has increased, we had a good choice of suppliers knocking on our door to show us their ranges. In the end a choice was made and the quality has remained high during the last two years of sales.Circuit board
The multi-layered printed circuit board was the most difficult item to develop as we wanted all the electronic components and connections to the outside world on a single board only slightly larger than the touchscreen. This task did not phase Chris Knight, our electronics engineer, who put in many hours working on the pcb design software to get the SMT components and tracks right on the multi layered design before it went out to prototype manufacture. The end result was a triumph for the design team.
Fig 2. New AshGraffix multi-layered single printed circuit board with new processor and using many surface mount components (SMT).Coding
The next task that faced Kevin Corcoran, our intrepid software engineer, was to incorporate all our ideas into code. After many discussions and updates during 2014 (usually accompanied with tea and some form of cake - this was before the Great British Bake Off was aired to the nation, as we found this was the best way to inspire creativity), we designed the home screen, menus and navigation through the system with the question to the operator:
What do you want to do today?
The concept was to give the operator access to commence ash sampling with two taps from power up screen and standard icons used for configuration and saving files to the unit itself or to USB output.With additional processing power, came new features:
Fig 3. Ash Probe and AshGraffix complete system.
During the first half of 2015 we were happy with the final design; software bugs had been eliminated and some test users had put the unit through its paces; only then did we go to the mining shows and offer the unit to the market.In conclusion, the key take aways for updating the display unit were:
Gary Wain is Product Manager, Parker Hannifin Manufacturing Ltd, Bretby Gammatech, Instrumentation Products Division, Europe
Innovations in the design of primary isolation valves and manifolds for mounting pressure instrumentation can deliver enormous pressure control advantages to both instrument and piping engineers, ranging from significantly enhanced measurement accuracy, to simpler installation and reduced maintenance. Parker Hannifin has created a comprehensive range of instrument manifold mounting solutions for the main types of pressure instrumentation, employing close-coupling techniques which eliminate impulse lines and tube fittings to improve overall instrument performance and reliability.What is close-coupling?
There is no formal definition for close-coupling, but it has come to mean any instrument mounting system that enables a user to connect an instrument directly on to the process line, and primary flow control isolation valve. The overriding objective of this is to optimise the accuracy of measurement, by eliminating the long runs of tubing, tube fittings and bends and joints between process pipe and instrument that can cause pressure drops, and gauge/ impulse line errors.
Transmitter ‘hook-ups’ are often configured individually for each application, and can be large, heavy and difficult to install. By replacing such arrangements with purpose-designed close-coupled manifold/mounting solutions, users are able to optimise accuracy and reap a whole range of additional benefits such as...
‘Hook-ups’ for pressure transmitters often involve the custom configuration of complex arrangements of tubing, with multiple connections and valves. Measurement errors can be introduced as a result of long length impulse lines. These errors are frequently compounded by the use of different tube, fitting and valve components whose diameters may vary throughout an instrument installation.
Inaccuracies can distort the pressure impulse signal, causing errors of up to 15% (on flow measurements).Traditional ‘hook-up’ for a differential pressure transmitter
This traditional solution uses two sets of valve assemblies to create the double block and bleed valves, which are connected with impulse lines and connectors to the instrument manifold. It involves numerous discrete components, with all the associated costs and assembly time, and introduces bends that cause attenuation and turbulence that can affect measurement accuracy. If not carefully specified, other measurement accuracy problems can arise from differences in bore diameters of the various components, and unequal lengths of tubing.
The close-coupled alternative
Jim Breeze is Product Manager, Instrumentation Connections and Process Valves, Parker Hannifin, Instrumentation Products Division, Europe.
There are many different industries that work with pressures over 10,000 psi, ranging from the oil and gas industry to laboratories and waterjet applications. With such high pressures and often elevated temperatures involved, safety is critical and choosing robust, tested and certified tubing and cone and thread fittings is essential. In fact, engineers and specifiers should always start by identifying compatible tubing and cone and thread fittings that can withstand these severe conditions. Taking this approach not only ensures the safety and reliability of the process but ensures that buyers have a clear specification that enables like-for-like comparison.
Parker Autoclave Engineers
When our company was established some 70 years ago, one of our major markets was laboratories. Today our pressure vessels can be found in laboratories all over the world and are renowned for their ability to withstand extremely high pressures and temperatures. The parameters we work within are between -423oF to 1200oF and full vacuum to 150,000 psi using over 40 different metals to-date.
Over the past seven decades, Parker Autoclave Engineers technology has been used in an extensive range of applications from providing tubing and cone and thread fittings for fueling rockets for the space industry to supplying the same for use on oil & gas wells in the deepest oceans.
Over the years, new markets have emerged that Parker Autoclave Engineers has been able to service, for example, the plastics industry has grown significantly in the last 30 years manufacturing producing LDPE (low density polythene) plastic being produced by the petrochemical industry to make food wrap, grocery and refuse bags. The raw LDPE plastic is made at very high pressures which we help to control.
The waterjet industry is another growing market where our expertise in producing high pressure tubing and fittings is used. Waterjets are employed for a huge variety of applications today from cutting lettuce in a farmers field, to slicing cake, chicken, meat or fish, to cutting gaskets, carpet, marble and even steel up to 10” thick. Often the waterjet industry requires pressures of over 60,000 psi and our team at Parker Autoclave Engineers are continually developing new technologies that can address these ever more challenging requirements.
A closely related industry to waterjet is waterblast. This involves wide high pressure or rotating streams of water, which are supported by our tubing and fitting systems. Waterblast products are used in a variety of applications from removing paint from the hull of a ship and immediately ready for repainting, to cleaning rubber off aviation runways. And in even higher pressure applications it can be used to break up concrete on a highway, without damaging the rebar, ready for repair.
There are many good environmental reasons to use this technology, which has resulted in its ongoing growth and importance in the marine, aviation, construction, civil engineering and quarrying industries. New uses are continually being identified, opening it up for a number of other sectors.
In most cases, we supply directly to the OEMs (Original Equipment Manufacturers) who recognise the fact that we have extensive capabilities and offer high quality products, with more connection choices available than any other manufacturer globally. This has led to us becoming a market leader in all our key markets. As part of our service, we provide a consultative and technical supporting role for our customers, ensuring their staff are fully trained in the use of high and medium pressure tubing and cone and thread fittings when operating at high temperatures and high pressures for a wide variety of applications.
The variety of uses for high and medium pressure tubing and cone and thread fittings is just about endless and you will find Parker Autoclave Engineers components in an extensive range of applications globally, where safety and performance are essential. View the Parker Autoclave product range here.
Michael O’Keane – Product Marketing Manager for Parker Autoclave Engineers.
For over forty years, Parker has the lead the development of the chemical hardening process ideal for ferrules designed to grip and seal stainless steel tubing. The process is called Suparcase. This article reviews the importance of metallurgy and how Parker has utilized Suparcase technology. The best compression tube fittings balance hardness, strength, and corrosion resistance. Parker's Suparcase ferrule-hardening process does not require the high temperatures and long duration of more-conventional case-hardening procedures that, in turn, lower stainless steel's corrosion resistance.
Stainless-steel compression tube fittings make it easy to install and maintain measurement and control instruments used in chemical processing, petrochemical plants, and many other industrial settings. They seal a broad range of aggressive fluids and chemicals, and resist internal and external corrosion. The fittings grip and seal by compressing the nose of a ferrule into the tubing OD. High-quality compression fittings hold internal pressure without leaks or failure until the tubing fractures. And users can repeatedly disassemble and reassemble them with no loss of sealing integrity.
Today, compression tube fittings are available from many fluid system technology suppliers, and they tend to look the same although they may vary slightly in design details and manufacturing processes - but looks are deceiving.
The ferrule, perhaps the most-critical component in compression tube fittings, appears rather simple. Yet it is highly engineered and, to function properly, requires considerable design, metallurgy, and production expertise. Not all products on the market meet these stringent requirements. For instance, the ferrule must precisely deform elastically and plastically during fitting assembly to properly grip and seal the tubing. Its front edge must be harder than the tubing to grip and seal through surface scratches and defects, but if the entire ferrule is too hard, it may not deform properly. Therefore, only the gripping edge of the ferrule is hardened while the rest has different, tightly controlled mechanical properties. Also, the hardening process must not compromise stainless steel's corrosion resistance. And finally, production processes must consistently turn out defect-free ferrules that hold tight tolerances and maintain metallurgical specifications.Design Evolution - One Ferrule to Rule them All
This article focuses on single-ferrule compression fittings, but many of the principles also apply to two-ferrule compression fittings. Ferrules were originally machined from cold-drawn stainless-steel bar stock. Cold drawing strain hardens the metal and imparts mechanical strength throughout the ferrule. But the ferrule's front edge was often still not hard enough to seal against tube surface defects such as scratches, weld seams, ovality, and hardness
One solution was to plate ferrules with a soft metal (such as silver) for a better seal when dealing with high-pressure gas. This improved resistance to impulse pressures, temperature swings, and vibration. Many ultra high vacuum and high-pressure seals deform hard edges into soft metal gaskets. Deforming the soft component with a hard one provides intimate metal-to-metal contact over the contact surfaces and overcomes surface irregularities. (A good source of detailed information is Industrial Sealing Technology, H. Hugo Buchter, John Wiley and Sons, 1979.) Manufacturers applied this concept to tube fittings by case hardening ferrules, which substantially increases surface hardness and lets them shear through surface defects and compensate for tubing variations.
Conventional gas nitriding case hardens the inner surface to a depth of approximately 0.004 in. During assembly, the ferrule front edge shears into the tube. If disassembled, the ferrule remains tightly locked to the tubing, allowing remakes with consistent sealing integrity. The fitting handles internal pressures, impulse pressures, temperature changes, and vibrations until the tubing fractures or fails in fatigue. However, gas nitriding (as well as carburization and carbonitriding) substantially lowers stainless steel's corrosion resistance. Process refinements let manufacturers harden only a band approximately 0.050 in. from the ferrule nose — sometimes termed a "limited nitrided" ferrule. This reduces the likelihood of corrosion, as the nitrided band is buried in the tubing surface. But it still poses a potential corrosion problem if, due to improper make up or surface defects, chemicals contact the band. Also, uninstalled fittings stored in corrosive environments, such as salt air, sometimes rust on the nitrided band.Case Hardening
Conventional nitriding and carburizing require high temperatures for the hardening constituents, nitrogen and carbon, to penetrate the passive oxide layer that makes stainless steel corrosion resistant. The high temperatures permit chromium, an anticorrosion alloying element, to diffuse through the metal and form chemically stable nitrides and carbides. These compounds give the surface layer most of its hardness, but in this chemically combined form chromium no longer resists corrosion, and the nitrided or carburized layer corrodes in many environments, including seawater and even moist air.
In addition, nitriding and carburizing can "sensitize" austenitic stainless steel exposed to high temperatures for an extended time. Carbon, which has low solubility in stainless steel, precipitates as chromium carbides in the grain boundaries, depleting regions adjacent to the grain boundaries of the chromium necessary for corrosion resistance. This process is known as sensitization.
A new hardening process that was introduced by Parker Hannifin in the late 1980s does not reduce the corrosion resistance of stainless steel. More recently, some other fittings manufacturers have introduced ferrule-hardening processes with similar advantages.
These new processes do not require the high temperatures and long durations that permit chromium diffusion. This keeps chromium in solid solution as a corrosion-resistant alloying element. The hardened layer is continuous, free of defects and voids, as the process tends to fill surface inclusions and substantially reduce end-grain corrosion effects.
The new processes also do not affect the bulk metal. There is no sensitization or change in mechanical strength beneath the hardened layer. The ductile layer deforms with the ferrule during assembly without cracking or spalling.
In these processes, carbon supersaturates the hardened layer. Carbon atoms occupy interstitial sites in austenitic stainless steel's face-centered, cubic crystal lattice, strengthening the hardened layer. The hard crystal-lattice structure would like to expand to accommodate the carbon atoms, but is constrained by the unhardened substrate. As a consequence, high compressive stress further enhances hardness. Compressive stress has the added benefits of substantially increasing a ferrule's fatigue and stress-corrosion resistance.
In general terms, the process removes the passive oxide layer from the steel surface, letting carbon atoms diffuse directly into the metal lattice without traversing the passive layer barrier. The carbon atoms diffuse at lower temperatures than other alloying elements, thus avoiding problems caused by formation of carbides and nitrides.Mechanical Action
A balance of metallurgical properties is critical to a ferrule's mechanical action during fitting assembly. For instance, the front edge of Parker Hannifin's CPI single-ferrule fitting shears down into the tubing, while the body arcs and clasps the tubing at the trailing edge. The front-edge grip prevents blow-out under pressure.
The ferrule must also work equally well across the tubing diameter tolerance range, typically ±0.005 in., and handle surface defects such as scratches that may be several thousandths of an inch deep. The arcing action turns the ferrule into a spring of sorts, letting it maintain tension against the tubing and the proper seat angle to seal despite vibration, mechanical shock, and thermal expansion. The back of the ferrule also loosely grips the tubing, damping vibrations that would otherwise transmit to the sealing interface.
Mechanical properties such as yield strength and hardness must be precisely controlled to effect this action. An extremely hard ferrule will be too stiff during assembly and will not bow and properly grip the tubing. But if it is too soft, the underlying material will not support the case-hardened surface. The result is an eggshell effect: the gripping front edge collapses during assembly and cannot hold the tubing under pressure. It also reduces the arcing spring effect.
Cold working is the only way to increase hardness and strength of Type 316 austenitic stainless steel after annealing. However, work-hardening rates change with the steel's composition, and constituent percentages can vary within an allowable range. Cold working can also reduce corrosion resistance. Thus, manufacturers must precisely control composition to maintain consistent mechanical properties and retain the austenitic structure, and case hardening must not uncontrollably change these.Lubrication
Stainless-steel parts that rub together under high pressure have a strong tendency to cold weld and seize. And to form high-integrity, leak-free tubing connections, ferrules must only slide forward during assembly and not rotate with the nut. To prevent seizing and ensure only linear ferrule movement, engineers must precisely control surface conditions and lubrication at the nut/ferrule and nut/body interfaces.
All mating surfaces must be smooth and free of defects, which exacerbate seizing. A bonded molybdenum-disulfide coating is the recommended lubricant for many compression fittings. Solid molybdenum disulfide readily adheres to surfaces, is noted for its lubrication and anti-seizing properties, and the solid does not squeeze out like liquid or soft, waxy lubricants under extreme pressure. The result is low assembly torque and consistent performance, even with repeated remakes.
Article contributed by Jim Breeze, product manager, Instrumentation Connections and Process Valves, Instrumentation Products Division Europe.
In industrial applications where high pressures (over 6,000 psi) are used, safety is of paramount importance. Even well below these pressures, precautions are needed to ensure a safe working environment. Tubing from different manufacturers may look the same on first glance – and, indeed, it may be certified to meet minimum requirements, but is it going to perform correctly, do the job and ultimately, not risk the safety of plant and personnel?Safety and efficiency – the Autoclave ethos
Because of the risks with connection leaks, there is a real need to make sure it is has undergone all necessary tests, is specified alongside appropriate fittings and will, therefore, optimise safety and performance.
To reassure customers that our tubing is certified over and above current requirements, we introduced our own AES 222 (Autoclave Engineers Specification). This covers an extensive range of additional requirements for which the tubing is tested, including bore finishes, milled sections, and non-destructive testing. The bore examination that we carry out, for example, involves the tubing being cut and then sectioned to allow microscopic examination of the bore finish.
Another important consideration with tubing is its ovality and wall thickness. If this is not rigorously tested, then uncertified, untested tubing will have different dimensions. This could lead to leakage and unnecessary safety risks.
It is therefore important to ensure that outside dimensions are specified, to ensure a correct fit of tubing. If the manufacturer operates to tight tolerances on tubing wall thickness and run-out it will ensure a more uniform wall thickness. This provides an even wall at the sealing point and leads to higher calculated design pressures.
These rigorous tests ensure that when the tubing is installed for the first time, risks are much reduced. Leakage is the biggest worry with any tubing system and our tests ensure that risks are kept to a minimum
What is sometimes overlooked is the fact that warranties for fittings are often not valid if the associated tubing is not specified from the same manufacturer. The reason for this is that another manufacturer’s tubing may compromise the safety of the fitting, as Parker Autoclave Engineers’ tubing and fittings are designed to work as a complete system. Our AES specifies and controls both material and critical dimensions and tolerances to meet the conditions for which they are designed.
So it can clearly be seen why safety should be a major concern when specifying tubing for industrial, high-pressure applications. By selecting tubing that has been rigorously tested, there is much less risk of tube leakage or failure, which compromises the safety of the whole system.
Franck Grignola is product manager, Autoclave Engineers, Parker Hannifin manufacturing, Instrumentation Products Division Europe.
History is littered with examples of how cutting corners has led to near or actual disaster. Despite advances in instrumentation and tubing, small bore tubing assemblies still represent the second largest source of hydrocarbon releases from offshore installations.
There are around 45 million tube fitting assemblies installed in the UK and the North Sea alone. Statistics show that as many as 26% of these may have the potential for allowing a leak or worse. Furthermore, 20% of all reported leaks are related to small bore tubing assemblies. Around 11% of these cases are classified as major severity events, yet there is a culture to install product manufactured at the lowest cost by sub optimum vendors and install with low grade, essentially unskilled and untrained operators.
There are also environmental consequences to consider, with fines now running into millions for any breaches, the simple fact is that specifying a lower cost product and installation practices may not be the most prudent financial decision in the long run.Drive towards lower costs can compromise safety
An important reason for this unacceptable situation may well be the decision to select and install product being made purely for cost reasons, without proper consideration of test data or long term performance. This can result in inferior materials or workmanship and/or untested designs being selected, which can seriously impact on safety. Often a manufacturer, even European-based and relatively credible at first glance, can recommend a product to an EPC or end customer without taking into account the demanding nature of certain applications in the oil and gas industry. Validating the credentials of manufacturers and contractors right at the start is essential to avoid issues further down the line.
One reason that some companies are able to produce valves and fittings at a lower cost may be because they have not invested in years of engineering leadership, research and development, metallurgists and rigorous testing and they do not have the backup of a strong global technical resource.
Correct performance at recognised boundaries may not be as critical if these products were being used in less demanding applications but in the offshore oil and gas industry, there is zero margin for error when it comes to process integrity.
The main considerations when specifying fittings and valves for critical applications, where safety is a priority, include a detailed review of competency with respect to the selected vendor. Do they have, for example;
Finally, can these companies offer accredited end user training and support covering, for example, such activities as;
The Parker Instrumentation training and support program is highly valued by customers who appreciate that it ensures they are best equipped to ensure a safe and secure installation that will give many years of reliable and long service.True cost
Low cost at specification stage may often result in unexpected and high costs later in the project or after project completion. This is not only disruptive to business and a potential environmental issue but can cost companies dearly in terms of damage to a brand and its reputation. This can result in corporate litigation or corporate manslaughter charges where operators are injured. There are also the replacement costs of failed systems and components to consider. The true lifetime asset cost of poor specification with inferior products can, therefore, be extremely high.
Involving Technical Managers/Engineering Managers in the decision-making process is essential to ensure that safety is not compromised.
Oil and gas companies should also ensure that instrumentation, tubing, and valves are correctly installed by competent and fully trained installers, in order to minimise risks. Buyers need to be supported in their bid to achieve the best value and that means not compromising on quality. In these safety critical applications, it is essential that selection of suppliers and products is based on quality and whole life costings.
Spencer Nicholson is Parker’s Division Innovation & Technology Manager, Instrumentation Products Division Europe
Fittings, Materials and Tubing Guide
Lost and unaccounted for natural gas, particularly at pipeline custody transfer points, is becoming a focal point for both buyers and sellers. Even somewhat small measurement error can result in very large economic gains or losses at current natural gas prices. One relatively large source of lost and unaccounted for natural gas is due to pulsation at the orifice meter-induced by compressors, flow control valves, regulators and some piping configurations. This article discusses some historical research and findings surrounding the topic of pulsation. In addition, we will provide some methods of measuring, monitoring and potentially correcting various types of pulsation supported by relevant examples.Background
In recent years the Pipeline and Compressor Research Council (PCRC), now known as (GMRC) Gas Machinery Research Council and a subsidiary of the Southern Gas Association, commissioned and funded various pulsation research projects at Southwest Research Institute (SWRI) in San Antonio, Texas. The PCRC sponsored research programs concluded that pulsation induced measurement errors to fall into two broad categories:
SRE directly relates to flow measurement error and therefore a very important topic to those who buy and sell natural gas. This paper will focus on methods to measure and quantify Square Root Error and subsequent Gauge Line Error, while also recommending several techniques to reduce pulsation effects on natural gas measurement.Square root error
Most natural gas flow measurement in the United States is performed by measuring pressure drop at two points (pressure differential) induced by an orifice plate. The gas flow rate (Q) is calculated using the basic formula Q = K√ΔPXP. The fixed orifice coefficient (K) is derived from a formula found in the latest edition of AGA Report Number 3. Differential pressure ΔP and line pressure P are measured either using mechanical chart recorders or electronic transmitters, remotely or directly mounted to the pressure taps, using a configuration of instrumentation valves, manifolds, and tubing.
Under steady-state flow conditions, gas flow rates can be accurately measured with current state-of-the-art equipment, including highly accurate pressure transmitters and flow computers. Despite the high degree of accuracy of current electronic measurement devices, inaccurate measurement still occurs when the ΔP modulates, or changes, at a frequency greater than the frequency that the measurement system extracts the square root of the ΔP.
This type of measurement error is called Square Root Error (SRE) and is the calculation of unsteady flow using the square root of the average P versus the average of the square root values of the instantaneous ΔP.
Pulsation from gas compressors, control valves, pressure regulators, and some piping configurations are one source of frequent ΔP modulation. Figure 1 is an excellent example that illustrates the amplitude and frequency of pulsation generated by a reciprocating compressor and a control valve. Three separate pulsation peaks are occurring in this system.
Figure 1: A graph exhibits three separate pulsation peaks. The operator can isolate the pulsation source by using new software filtering capability and modifying conditions in the new field. Minimizing and eliminating the pulsation source can ultimately improve the meter’s measurement accuracy.
The field technician operating the SRE Indicator is typically able to isolate the pulsation source(s) by using new filtering software and modifying the field conditions to generate new responses. This enables the operator to make necessary field changes that should improve measurement accuracy.Other primary element errors
SRE is the largest component of pulsation induced primary element error. However, inertial error and the coefficient shift will both increase in magnitude under extreme pulsation conditions. A brief explanation of each follows:
Pulsating gas flow will tend to remain in motion due to its inertia. As a result, flow velocity changes lag behind ΔP changes. Inertial errors are insignificant unless pulsation amplitude and frequency are both
Though difficult to quantify, test data indicates that pulsation levels above 1.5% SRE contribute to shifts in the orifice coefficient.
The %SRE is measured at operating conditions and is used to approximate the primary element error induced by pulsation and to determine whether corrective action is necessary.
Percent Square Root Error (%SRE) is measured with a device manufactured and marketed by Parker called the Square Root Error (SRE) Indicator. This analytical instrument utilizes a high-frequency response ΔP transducer and software to calculate %SRE according to the formula developed by SWRI, illustrated earlier in this paper.
The SRE Indicator is used by field technicians to measure the severity of pulsation and calculate %SRE. The results can be used to determine if corrective action is necessary. However, because other primary element errors (inertial error and coefficient shifts) are not directly measured, %SRE should not be used to correct flow measurement readings.
Measurement error caused by pulsation at custody transfer points can create large economic discrepancies between natural gas buyers and sellers. Therefore, many natural gas purchase contracts contain language that set limits on %SRE (sometimes as low as 0.20% SRE) and typically place the burden of reducing or eliminating pulsation on the seller.
The simplest method of reducing pulsation induced SRE is to raise the ΔP by changing the orifice plate. Unfortunately, this may also limit the operating range of the measurement system.
In some cases, the piping system could be modified or the pulsation source could be moved to reduce SRE. This can be time-consuming and costly.
Another popular corrective action for high SRE is to install a device, such as a restricting orifice, between the pulsation source and the measuring station. However, these restricting devices can result in higher compression cost and a limited flow range. %SRE can also be reduced by installing an acoustic filter to remove most of the pulsation. Although more costly than a restricting device, a properly designed acoustic filter will operate over a much wider flow range with a lower pressure drop.Gauge line error
Gauge Line Error (GLE) exists when the differential pressure (ΔP) at the tips does not equal the differential pressure (ΔP) at the end of the gauge lines. GLE is typically caused by either pulsation or other flow phenomena.
The gauge line starts at the orifice taps and ends at the transmitter, flow computer, or chart recorder connections. It includes any pipe fittings, valves, valve manifolds, tube fittings, instrument tubing, and condensate chambers or bottles that may be installed between the orifice taps and the measurement device.
Research conducted by SWRI determined that gauge line error has two components:
Parker developed its initial GLE Indicator in 1990, following it in 1996 and 2005. The current SRE/ GLE Indicator includes the ability to perform both %SRE and GLE tests, thus measuring and quantifying both gauge line error and square root error.
Figure 2: SRE6 and GLE6 test equipment that enables an operator to perform an SRE and GLE test simultaneously.
The GLE Indicator compares the differential pressure at the orifice taps with the differential pressure at the end of the gauge lines. Any difference between the two signals would be associated with gauge line error.Testing results
Extensive field-testing with the GLE Indicator confirmed the research conducted at Southwest Research Institute (SWRI) by PCRC. The lab test examples should provide a better understanding of GLE issues and measurement problems resulting from incorrect transmitter mounting practices.
As noted previously, numerous gas contracts now include pulsation magnitude clauses and many transmission companies require the installation of acoustic filters to minimize pulsation levels and %SRE. However, GLE tests conclude that gauge line error may continue to be present even after the installation of an acoustic filter and despite %SRE readings as low as 0.1%.
System complexity and numerous dependent variables, including pulsation levels, gauge line lengths, gauge line diameters, operating pressure, gas density, and gas velocity make it extremely difficult to observe a measurement location and predict what gauge line error, if any, will be present. GLE testing is currently the only recognized method to determine the presence of gauge line error.
Proper installation of the transmitter and/or electronic flow meter (EFM) in a manner that minimizes or eliminates gauge line error by removing as many of these dependent variables as possible is the best option.
Best practices include:
Using a short length of 1/2" O.D. instrument tubing and full opening quarter turn ball valve between the orifice fitting and measurement device creates numerous mating of female NPT connections and small “volume chambers,” which could create gauge line shift (pulsation rectification effects).
“Best practices” suggest using a system that directly mounts and closely couples the measurement device to the orifice taps. This method continues to gain wide acceptance within the industry illustrated by over 10,000 installations currently in service.
Figure 3. Parker’s Direct Mount System. The manifold system reduces the effect of Gauge Line Error on the total measurement system. Note the reduced number of leak points and sensing line length, and the uniform diameter between the orifice ports and the measuring elements.Summary
Pulsation created by compressors, flow control valves, regulators, and some piping configurations may create unacceptable levels of Square Root Error (%SRE) and/or the resulting Gauge Line Error (GLE).
Pulsation at the orifice meter is a major source of lost and unaccounted for natural gas, which can create large economic gain or loss for both buyers and sellers along with a natural gas pipeline system.
%SRE and GLE can be measured and quantified using an SRE/GLE Indicator to verify measurement accuracy at a specific time and place. Pulsation and resulting high % SRE creates a high probability that GLE is present. Volume chambers or numerous measurement devices connected to the same set of orifice taps may compound or create GLE.
Transmitters or EFM should be close coupled to the orifice taps with equal length, large bore (0.375" I.D. or greater), constant diameter gauge lines to minimize or eliminate GLE; however, this process will not reduce or eliminate %SRE. The pulsation source must be eliminated, piping systems modified, ΔP increased, a restricting device installed, or a properly sized acoustic filter installed to reduce pulsation and resulting %SRE.
Article contributed by BJ Jackson, Product Manager - PGI Specialized Systems, Parker Hannifin, Instrumentation Products Division.