A new valve concept, developed under a GMRC program by Southwest Research Institute (SwRI) and co-funded by BP, uses electromagnetic actuators to control valve-plate motion and create a soft landing at both 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 tested at Southwest Research Institute. The latest version, manufactured by Southwest Research Institute and Cook Compression, is currently operating in a long-term field test at a U.S. BP production site. The following discusses the SAV product development process, laboratory and field testing logistics, site preparation, test execution, and test results. Measurement data for SAV performance, such as valve-plate impact velocities, damping effects, and valve-motion profiles are presented. The capacity-control capability of the SAV is demonstrated.

Operation of a reciprocating compressor is closely linked to the performance of its cylinder valves. Compressors traditionally use passive valves to control the suction and discharge flow process of its 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 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 identified early on in the program was a semi-active compressor valve (SAV). This technology has the potential to improve both plate-valve life and efficiency, while minimizing failure risk traditionally associated with use of fully active compressor valves.

The life of a conventional reciprocating compressor plate valve is typically four to eight months for severe 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.

Environmental failures can usually be prevented by the proper choice of valve material and gas-stream conditioning, whether filtration, separation, or other.

On the other hand, mechanical causes result from high-cycle fatigue and abnormal valve—-plate mechanical motion, related to high valve lift, valve operation at off-design conditions, plate flutter, pulsations, 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 valve design and materials data. Also, valves are designed for a single optimal operation point; hence, valve life is often reduced when operating conditions deviate significantly from the design point.

In the traditional compressor-valve design, increased valve life (reliability) directly relates to a decrease in valve efficiency. This is due to the increase in valve lift (and flow-through area) being limited by the corresponding increase in valve-plate impact force. As plate impact velocities increase due to higher valve lift or valve operation at off-design conditions it causes excessive impact stresses and valve plate accelerated material damage/fatigue rate. 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 plate damping. Reducing plate impact velocity greatly increases life of a valve plate and springs. Clearly, lack of durability and low efficiency of the current-technology passive-valve designs demonstrates the need to control valve motion.

Semi-active valve concept

To address this need, SWRI engineers developed a new valve concept for electromagnetic damping, creating a soft landing at both the valve seat on closing and at the valve guard on opening. A conventional plate valve design is used but the valve springs are replaced or augmented with electromagnetic coils and actuators for position sensing and to provide a controlling force prior to the plate’s impact. This 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., while motion of the plate is actively controlled, pressure sensors or shaft encoders for control are not required). Further, should any control mechanism fail, the valve assembly continues to act as a passive valve.

Figure 1 shows the first schematic of the SAV concept. The basic SAV functional principle is that control is required, for only a few milliseconds, at two points in the valve’s plate motion cycle: just prior to opening impact and just prior to closing impact. Pressure forces 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: plate velocity is reduced to dampen its impact on the guard and avoid plate bouncing. (Here the plate absolute position is measured using optical probes that have a voltage-to-distance proportional output and time is related to crank angle.) By controlling plate motion prior to guard and seat impacts, electromagnetic control power requirements are minimized, and valve performance is not significantly affected.

Previous research work within this program demonstrated that life of a compressor valve is significantly enhanced by reducing plate impact kinetic energy. The SAV accomplishes this by actively limiting the plate’s guard/seat impact kinetic velocity. Furthermore, since impact velocity can be controlled actively, lift can be increased, which increases compressor capacity. This not only delivers increased throughput, but allows the operator to selectively optimize compressor operation for improved life versus increased capacity to match unique production requirements. The valve self-regulates to the compressor operating point of the compressor as sensing is based on valve-plate movement only (i.e., if compressor speed changes, the valve will follow the speed in real time without operator input required). Finally, the valve is intrinsically fail-safe, as it reverts back to passive operation if the electronic controls or mechanical parts should fail.

Development and installation

Initially two laboratory prototype semi-active valve models were developed to demonstrate concept feasibility and function. Laboratory, mechanical fatigue, and closed-loop tests clearly demonstrated concept viability, and 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 U.S. BP production site.

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

The site selected by BP for the field testing of the SAV is within a gas-gathering production facility in Oklahoma. The unit 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 severe duty in upstream gas-gathering compression application, compressor valves typically are replaced frequently. Specifically, as compression ratios are high and natural gas often contains sand, corrosion products, hydrocarbon liquids, and soapy water from production wells, conventional-valve life is significantly reduced. The unit selected for the tests is critical for gas production. Downtime for installation, commissioning, and start-up testing had to be limited to less than 12 hours. This unit operates continuously with very few scheduled shutdowns. 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, unit running speed cyclically varies between 1,100 and 1,350 RPM.

Electrical installations at the facility had to meet NFPA 70 Class 1, Division
2 requirements, which significantly affected 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.

Because of the customer 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 open cable tray to a weatherproof NEMA enclosure outside the hazardous area that contained the SAV controllers which are power supplies.

As electrical wiring, boxes, and enclosure were pre-installed, actual SAV installation into the compressor required approximately 4 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 SAV functioning. 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.

Testing and performance

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

Figure 4 shows a saw-tooth controller output voltage that is triggered when the plate velocity exceeds a specified value, causing the actuators to apply an opposing (dampening) force on the valve plate. This results in plate deceleration just prior to guard or seat impact. The process is repeated for every cycle of the plate motion and, thus, the SAV operates continuously with low impact-plate kinetic energy and material stresses. It should be noted only the opening impact is controlled as the closing impact in this case was relatively soft.

Additional tests performed validated the SAV’s ability to control plate motion. These tests also served to verify SAV potential for compressor flow control (i.e., if the SAV can be used to increase or reduce the suction valve plate’s opening or closing period by advancing or delaying the plate motion, then cylinder flow capacity can be affected). For example, the cylinder flow capacity can be reduced by delaying 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 that 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.

As of April 14th, 2009, the four SAVs have run without interruption for nearly 12 months, accumulating over 8,500 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. This has 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.

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 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%.

It is important to note that the current version of the SAV controller, as installed at the BP test site, was not designed for and provided only limited capacity control. Nonetheless, it clearly demonstrated the feasibility of capacity control using SAV, and future versions of the controller will be designed with capacity control functionality built in.

Final words

Test results clearly demonstrate that the SAV concept is practical and 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 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.

Field testing of the SAVs is ongoing As a next step, four additional SAVs are currently being built to be installed on the discharge side of the compressor at the same BP gas gathering compression facility. n

Acknowledgements

The authors would like to acknowledge

the Gas Machinery Research Council, the United States Department of Energy, and BP Exploration & Production for their financial and technical support of this valve research program. Based on a paper presented at

the Gas Machinery Conference held in Albuquerque, New Mexico, October 6-8, 2008.

References

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