Soft-Landing Semi-Active Valve For Reciprocating Compressors Inspires Confidence During Field Tests

June 2009 Vol. 236 No. 6

Ongoing field testing of a semi-active valve (SAV) concept on a BP natural gas gathering facility reciprocating compressor through Oct. 3, 2008, demonstrated that the concept is practical and can significantly extend compressor valve life. Specifically, the field tests showed:

  • In all tested operating conditions, reduction of the plate impact kinetic energy above 70% was achieved.
  • Without any operator input, the valve automatically self-regulated to any compressor speed and operating conditions.
  • The valve can safely operate in natural gas compression facility meeting Class 1, Division 2 (or Division 1) requirements.
  • The SAV is inherently retrofitable to existing compressor installations.
  • The valve reverts to passive operation and continues to function when the control mechanism fails or is disabled.
  • The SAV is capable of providing flow capacity control.

The success of the field testing is important because the single largest maintenance cost for a reciprocating compressor is compressor valves. Plate valve failures can primarily be attributed to high-cycle fatigue and sticking of the valve plate resulting in excessive plate impact velocities. Thus, controlling the valve plate motion and impact velocities can greatly increase the life of a plate valve while also allowing for improved capacity control of the compressor.

A new valve concept, developed under a GMRC program by Southwest Research Institute® (SwRI®) and co-funded by BP, uses electromagnetic actuators to control the valve plate motion and create a soft landing at both the valve seat and guard. This concept is referred to as a “semi-active valve” (SAV) since, although the valve still relies on gas forces for the plate to move, the device senses and then controls the plate motion using electromagnetic coils.

The SAV development program has evolved and matured through three prototype devices, each of which has been tested at Southwest Research Institute. The latest version of the SAV, manufactured by Southwest Research Institute and Cook Compression, first underwent functional testing at Southwest Research Institute and is currently operating in a long-term field test at a BP production site in the U.S.

Cost Savings

The operation of a reciprocating compressor is closely linked to the performance of its cylinder valves. These compressors have traditionally used passive valves to control the suction and discharge flow process of the compressor cylinder. However, valve failures are generally cited as the most common cause of scheduled and unscheduled compressor outages, and the single largest maintenance cost items on reciprocating compressors are valve replacements and repairs.

With the emergence of larger machines operating over wider speed and pressure ratio ranges over the last 15-20 years, this trend has worsened. Consequently, the industry has to consider improvements in valve technologies to be able to compete with alternative compression technologies.

To address these needs, the Gas Machinery Research Council, BP, and the Department of Energy funded a multi-faceted program to develop advanced reciprocating compressor valve technologies. One promising technology that was identified early on in the program was a semi-active compressor valve (SAV). This technology has the potential to improve both life and efficiency of plate valves, while minimizing the failure risk that has traditionally been associated with the usage of fully active compressor valves.

The life of a conventional reciprocating compressor plate valve is typically four to eight months for pipeline applications and usually significantly shorter for harsh upstream and process applications, often less than a week. Valve failures can be divided into two major categories: Environmental and Mechanical. Environmental causes are principally due to corrosive contaminants, foreign material, debris, liquid slugs, or improper lubrication. These environmental failures can usually be prevented by the proper choice of valve material and conditioning of the gas stream (filtration, separation, etc.).

On the other hand, mechanical causes are the result of high cycle fatigue and abnormal mechanical motion of the valve plate, related to high valve lift, valve operation at off-design conditions, plate flutter, pulsations, and/or spring failure. Some of these can be controlled by careful analysis and design of valve components (i.e., guard, seat, sealing elements, springs, and damper plates) for a fixed compressor operating point.

However, mechanical valve failures are generally difficult to control as they are principally related to valve internal mechanical behavior and material limitations – especially for the compressor operator, who has limited access to the design and materials data of the valves available for his machines. Also, valves are designed for a single optimal operation point; hence, valve life is often reduced when the operating conditions deviate significantly from the design point.

In the traditional compressor valve design, an increase in valve life (reliability) directly relates to a decrease in valve efficiency. This relationship is due to an increase in valve lift (and flow-through area) being limited by the corresponding increase in the valve plate impact force. As plate impact velocities increase due to higher valve lift or valve operation at off-design conditions, the velocities cause excessive impact stresses and an accelerated material damage/fatigue rate to the valve plate.

Also, above a certain impact velocity, valve plate failure is attributable to plastic deformation of the valve springs. These springs consequently fail to provide adequate damping for the plate. Reducing plate impact velocity can greatly increase the life of a valve plate and springs. Clearly, a lack of durability and low efficiency of the current technology passive valve designs demonstrates the need to control valve motion.

New Valve Concept

To address this need, SwRI engineers have developed a new valve concept that provides electromagnetic damping and creates a soft landing at both the valve seat on closing and at the valve guard on opening. The concept is to utilize a conventional plate valve design but to replace or augment the valve springs with electromagnetic coils and actuators that sense the position and provide a controlling force prior to the plate’s impact.

This concept is referred to as a “semi-active electromagnetic plate valve” because it is still passively activated by gas forces and only actively controlled prior to impact (i.e., although actively controlling the motion of the plate, this valve does not require pressure sensors or shaft encoders for control). Furthermore, should any of the control mechanisms fail, this valve assembly will continue to act as a passive valve.

Brun et al., 2006, described the functional principles of the semi-active valve in detail and, thus, the concept is only briefly outlined here. Figure 1 shows the first schematic of the SAV concept. The basic functional principle of the SAV is that control is required for only a few milliseconds at two points in the valve’s plate motion cycle: one just prior to the opening impact and one just prior to the closing impact. Pressure forces (just like in a conventional valve) control the remainder of the plate motion profile.

Figure 2 shows the desired functioning of the SAV on a typical (measured) valve plate motion profile: the plate velocity is reduced to dampen the plate’s impact on the guard and avoid plate bouncing. (Here the plate’s absolute position is measured using optical probes that have a voltage to distance proportional output and time is related to crank angle.) By only controlling the plate’s motion prior to the guard and seat impacts, the electromagnetic control power requirements are minimized, and the valve’s performance is not significantly affected.

Figure 1: Semi-Active Valve Initial Concept.
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Figure 2: SAV Plate Impact Control
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Concept Advantages

Previous research work within this program demonstrated that the life of a compressor valve is significantly enhanced by reducing plate impact kinetic energy. The SAV accomplishes this by actively limiting the guard/seat impact kinetic velocity of the plate. Furthermore, since the impact velocity can be controlled actively, the lift can be increased, thus, increasing the compressor’s capacity. This feature not only provides the operator with increased throughput but allows the operator to selectively optimize the compressor operation for improved life versus increased capacity to match his/her unique production/operation requirements.

The valve self-regulates to the operating point of the compressor as the sensing is based on the movement of the valve plate itself only (i.e., if the speed of the compressor changes, the valve will follow the speed in real time without operator input required). Finally, the valve is intrinsically (inherently) fail-safe as it reverts back to passive operation, if the semi-active valve’s electronic controls or mechanical parts should fail.

SAV Development

Initially, two laboratory prototype semi-active valve models were developed to demonstrate the concept’s feasibility and function. The design and the testing of these prototypes were described by Brun et al., 2006. As these laboratory, mechanical fatigue, and closed-loop tests were successful and clearly demonstrated the viability of the concept, GMRC and Cook Compression decided to jointly develop a beta production version of the SAV for full scale field performance and endurance testing at a BP production site in the U.S.

These early SAV laboratory tests also provided critical design and performance data required for the implementation of the beta production version of the SAV described here. Figure 3 shows a drawing of the SAV design, and Figure 4 shows a photograph of this design after the completed assembly. This valve underwent extensive mechanical testing at a SwRI laboratory testing to ensure proper functioning prior to installation at the field site.

Figure 3: SAV Design.
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Figure 4: SAV for Field Testing.
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Field Testing

The site selected by BP for the field testing of the SAV is within a gas gathering production facility in Oklahoma. The unit that was chosen for the SAV beta version is a 2,500 hp gas-engine driven two-stage Ariel machine in a well-facing application operating at a maximum operating speed of 1,350 rpm. The first stage compresses gas from 45 psi suction to 150 psi discharge pressure. The SAVs were installed on the suction side of the first stage (of two first stages) to replace four conventional 7-inch plate valves.

Because of the severe duty in upstream gas gathering compression application, compressor valves typically have to be replaced frequently. Specifically, as the compression ratios are high and as the natural gas often contains sand, corrosion products, hydrocarbon liquids, and soapy water from the production wells, the life of conventional valves is significantly reduced.

The unit selected for the SAV tests is critical for gas production and, thus, downtime for installation, commissioning, and start-up testing had to be limited to less than 12 hours. This unit operates continuously (24/7) with very few scheduled shutdowns and most unscheduled shutdowns are for valve replacements. Suction gas supply pressure swings of 15-20 psi are common and, to meet production and demand requirements, the unit running speed cyclically varies between 1,100-1,350 rpm.

Electrical installations at the facility had to meet NFPA 70 Class 1, Division 2 requirements, which significantly affected the design of wiring and conduit requirements. Although the SAV field test was scheduled for only six months duration, BP required that Permanent Installation Standards be applied, that all facility modifications be fully documented, and that BP’s rigorous safety and quality standards be met. Furthermore, as the SAV installation requires electrical wiring penetrating the compressor casings, the cable fittings and modified valve covers had to be hydro-tested per ASME standards.

Figure 5 shows the compressor with the SAVs installed. Because of the customer’s site installation requirements, semi-rigid mineral insulated (MI) cable to an explosion proof (EP) junction was used with the fitting through the valve cover being the primary gas insulation and the junction box providing secondary gas insulation. From the junction boxes, the MI cable was routed using an open cable tray to a weatherproof NEMA enclosure outside the hazardous area that contained the SAV controllers which are power supplies (Figure 6). The enclosure was purposely oversized as it was also used to store test instrumentation and tools.
(caption)Figure 5: Compressor with Set of SAVs Installed.
(caption)Figure 6: Enclosure Containing SAV Power-Electronics.
As all the electrical wiring, boxes, and enclosure were pre-installed prior to the test date, the actual installation of the SAVs into the compressor required approximately four hours. After purge and start-up, the compressor was initially run unloaded at 900 rpm for 20 minutes (recycle wide open) to tune the controllers individually and verify proper functioning of the SAVs. The compressor was then loaded and returned to its normal operating range (1,300 rpm, 3:1 pressure ratio). Performance, capacity control, and endurance field testing of the SAVs was thus initiated.

To validate proper functioning of the SAV, the input sensor signal, the control response, and the output signal to the actuators were monitored and recorded on a digital storage oscilloscope while control parameters were varied. The performance was tested for varying output signal voltage (actuator force), response delay time, response function width, and response trigger level. This testing was performed on all four SAVs to identify the optimal control parameter setting for valve plate impact damping while maintaining cylinder performance (i.e., the initial controller tuning was performed). Long-term performance is also being recorded using a digital oscilloscope.

Figures 7 through 9 show the input signal from the motion sensing coil and the control response to the actuators to demonstrate the functioning of the SAVs. Specifically, Figure 7 shows a sawtooth controller output voltage that is triggered when the plate velocity exceeds a specified value and causes the actuators to apply an opposing (dampening) force on the valve plate. This results in plate deceleration (Figure 8) just prior to the impact on the guard or seat.

The process is repeated for every cycle of the plate motion (Figure 9) and, thus, the SAV operates continuously with low impact plate kinetic energy and material stresses. It should be noted that in Figure 9 only the opening impact is controlled as the closing impact (in this case) was relatively soft.

A number of additional tests were performed to validate the SAV’s ability to control plate motion. These tests also served to verify the SAV’s potential for compressor flow control (i.e., if the SAV can be utilized to increase or reduce the suction valve plate’s opening or closing period by advancing or delaying the plate motion, then the flow capacity of the cylinder can be affected).

For example, the cylinder flow capacity can be reduced by delaying the closing of the suction valves. This effectively results in an infinite clearance volume during the discharge stroke for a short period of time (the SAV closing delay). The motion output signal traces in Figure 10, which were recorded during the field tests, demonstrate SAV capability of advancing/delaying the plate motion and, thus, its ability to provide compressor flow capacity control.

SAV Performance

As of Oct. 3, 2008, when this was prepared for the GMRC Conference, four SAVs had run without interruption for more than 25 weeks, accumulating more than 4,200 fully loaded operating hours each and performing within their design specifications with no single failure reported. Thus, the SAVs have demonstrated a valve life increase by a factor of at least five when compared to conventional valves run in parallel with the SAVs.

Clearly, this has resulted in reduced downtime at the field test site. Endurance testing is still ongoing, but regular cylinder performance measurements (PV cards) and diagnostics have shown no noticeable degradation of the valves. The cylinder end with the SAVs installed is currently operating near optimal efficiency.

Figure 7: SAV Sensor Signal and Control Response.
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Figure 8: SAV Plate Impact Velocity Reduction.

Figure 9: SAV Opening Impact Response.

Figure 10: SAV Capacity Control.

During the initial testing, the valve impact velocities and capacity control were also measured and quantified. Table 1 shows results for the plate impact velocity reduction for cases with the controller turned on and off (i.e., in semi-active mode and passive mode). It should be noted that impact velocities were measured from the sensor coil and have a measurement uncertainty of approximately 15%.

Table 1: Plate Impact Velocity Reduction
Speed (RPM)  No-SAV (m/s)  SAV (m/s)  Impact Energy Reduction %
900 (low load)   1.2 0.6 75
1,250 2.8 1.5 72
1,350 3.1 1.5 77

Table 1 also shows the resulting reduction of plate impact kinetic energy, which is directly related to plate peak material stress and, thus, plate fatigue life. Reductions of above 70% were easily achieved. Based on S-N curves for Peek Material, the SAV plates are theoretically operating in the range of infinite fatigue life.

Table 2 shows results from the functional tests for SAV capacity control. Here, flows are estimated using a cylinder performance analysis program as they could not be directly measured at the site. The uncertainty of this method is estimated to be approximately 25%.

Table 2: Capacity Control Demonstration – Delayed Valve Closing
Speed (RPM)  Valve Open: No SAV With SAV   Flow Change
900 (low load) 30 ms 26 ms – 6%
1250 18 ms 20 ms – 4%
1350 16 ms 18 ms – 4%

It is important to note that the current version of the SAV controllers, as installed at the BP test site, were not specifically designed for capacity control functionality and, therefore, allowed for only limited control over this feature. Nonetheless, they clearly demonstrated the feasibility of capacity control using SAV, and future versions of the controller will be designed with capacity control functionality built in.

Conclusions

Four semi-active valves (SAVs) were installed into the suction side of the first stage in an Ariel compressor at a BP upstream gas gathering facility for field endurance testing and performance validation. As of Oct. 3, 2008, the four SAVs have continuously operated for more than 4,200 fully loaded operating hours each, with no single failure reported. Test results clearly demonstrate that the SAV concept is practical and that a properly designed SAV can significantly extend compressor valve life. Specifically, the SAV field tests showed:

  • Reduction of the plate impact kinetic energy above 70% was achieved for all tested operating conditions.
  • The valve automatically self-regulated to any compressor speed and operating conditions without any operator input.
  • The valve can safely operate in natural gas compression facility meeting Class 1, Division 2 (or Division 1) requirements.
  • The SAV is inherently retrofitable to existing compressor installations.
  • The valve reverts to passive operation and continues to function when the control mechanism fails or is disabled.
  • The SAV is capable of providing flow capacity control.

The field testing of the SAVs is ongoing, and future publications will report on the final resulting SAV life. As a next step, four additional SAVs are being built for installation on the discharge side of the compressor at the same BP gas gathering compression facility.

Acknowledgements
This article is based on a presentation at the Oct. 6-8, 2008, GMRC Gas Machinery Conference in Albuquerque, NM. The authors acknowledge the Gas Machinery Research Council, the U.S. Department of Energy and BP Exploration & Production for their financial and technical support of this valve research program.

References
For references to literature associated with this article, contact the lead author at the Southwest Research Institute, San Antonio, TX. Dr. Klaus Brun, Manager -Rotating Machinery and Measurement Technology, Southwest Research Institute, San Antonio, TX 78238-5166, Fax: 210 681 9661, e-mail: klaus.brun@swri.org, www.gasturbines.swri.org.

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