November 2020

Valves, Pumps and Turbomachinery

Advances in compressor anti-surge valve design enhance reliability and performance

Centrifugal and axial compressors are some of the most critical components used within a process plant to handle gases.

Haines, B., Nelson, M. P., Flowserve Corp.

Centrifugal and axial compressors are some of the most critical components used within a process plant to handle gases. These machines impart energy to the process fluid, helping create optimum process conditions while also allowing the transfer of fluid. They typically comprise a major portion of the equipment cost on a project and generally do not have spare or standby equipment. Proper operation of these machines is imperative to ensure that the entire plant runs safely and reliably.

However, these compressors can be exposed to surge events, which tend to have adverse effects on a compressor’s lifetime productivity. In some cases, the surge event can also be disastrous to the compressor, leading to extended periods of lost productivity.

Compressors are designed to take in energy from an external source (compressor driver) and transfer it to the process fluid to maintain a continuous and reliable flowrate. However, if the flowrate changes suddenly due to an upset condition elsewhere in the system, the compressor’s energy balance is disrupted, causing a reversal of gas flow, higher temperatures and increased vibration inside the compressor. When this happens, the blade stalls, causing the thrust and flow across the blade to stop. The cyclic reverse and normal flow inside the compressor result in forces hammering the bearings of the compressor. This occurrence is referred to as a surge event. Together, these conditions can lead to catastrophic compressor failure. This process can happen in seconds, making it difficult to react to—which puts not only the compressor, but also the entire operation, at risk. The latter scenario faces a high potential of resulting downtime and production losses. Operators must employ methods to protect the system from the dangers of surge.

A standard method to mitigate these risks is an anti-surge control system that controls the performance of the compressor through an anti-surge valve (ASV).

How anti-surge valves work

ASVs recycle media from the compressor’s discharge end back into the inlet when surge conditions are detected. By temporarily increasing the amount of material into the compressor, the pressures equalize and a blade stall is avoided. After the feed rate stabilizes, the ASV closes and the compressor returns to normal.

ASVs must be able to adjust the flowrate quickly and precisely before the compressor crosses the surge line. While ASV designs vary by manufacturer, all must meet certain critical aspects, including:

  • Valves must be able to provide precision control and stroke quickly and accurately
  • They must be simple to set up, tune and maintain
  • Each valve’s capacity must be sufficient to prevent a surge event
  • Valves must be able to attenuate noise to acceptable levels in high-pressure-drop and high-flow conditions
  • The valve and actuator design must be robust enough to prevent it from damaging itself during fast stroke events.

ASVs are a very important part of a compressor system, but every valve does not contain the same internal components, so it is important to understand what these components do and whether they are required for a particular application.

Positioner feedback mechanisms enable precise controllability

Operators must be able to precisely control an ASV’s stroke position to account for frequent small upsets. This gives operators the ability to make minor adjustments when running close to the surge line, allowing for enhanced productivity.

To ensure proper control, the positioner must receive accurate feedback to determine the current valve stroke position. A typical positioner has a feedback shaft that rotates as the valve strokes. The starting rotation angle of the positioner feedback shaft indicates the valve is closed, and the ending angle indicates the valve is open. Any angle between the starting and ending angles represents a certain percentage of the total valve stroke.

The mechanical linkage between the linear motion of the valve stem and the rotary motion of the positioner feedback shaft commonly consists of a take-off arm with a pin that rides in a slot. For control valves with strokes of 305 mm (12 in.) or longer, the typical positioner take-off arm is long, cumbersome and sensitive to vibration. Over time, excess vibration can cause the positioner to fall out of alignment, preventing its ability to accurately calibrate the valve stroke position.

For long stroke lengths, a more reliable configuration couples the rotary motion of the positioner feedback shaft to a tube with a helical slot mounted between two bearings. The axis of the tube is parallel to the valve stem. A take-off arm with a pin is mounted to the plug stem. As the valve stem moves, the pin rotates the tube, providing plug stem feedback to the positioner.

Valve design determines flow capacity

While the compressor is slowing down during an emergency event, the ASV opens, providing gas to the compressor inlet that maintains high flowrates in the compressor, preventing surges during the shutdown. In this case, the valve needs to deliver very high flows to the compressor very quickly.

The ability of the valve to successfully deliver such high flowrates depends on its design. For example, an angle body valve with a large-volume capacity gallery is capable of providing more flow capacity than other valve designs. The design of the valve also determines the ability to accommodate severe service trims, which can help reduce noise and vibration.

Actuators determine stroke speed

During emergency situations, when the feedstock to the compressor has been interrupted, operators must be able to open the ASV quickly. Many oil and gas companies require ASVs to fully open within 1 sec, activated by solenoid. This fast stroke speed is critical to ensuring that operators have enough time to shut down the compressor and investigate the cause of the upset.

Actuators and control systems in an ASV determine how quickly the valve can open and close. Since diaphragm actuators cannot provide the thrust and speed required, pneumatic piston actuators are necessary. Pneumatic piston actuators have positioners that supply air to both sides of the piston. This four-way control design is integral to produce the amount of speed and thrust required to quickly stroke the valve.

The control systems used on an actuator vary from banks of traditional three-way flow boosters to complicated systems where a computer, some type of electronic position feedback, and a large spool and block are joined together to make a positioner.

Air cushions prevent damage to actuator and valve assembly

While ASVs must be able to stroke quickly, over time, this ability can cause damage to the actuator and valve assembly. For actuators with long strokes and fast stroking times, there is a need to decelerate and diffuse high-impact energies at the end of the stroke to avoid impact and potential damage to the actuator and valve assembly. For anti-surge applications, large valves that have fast stroke speeds and long stroke lengths use air cushions on the top of the cylinder to protect them from impact in the open direction. Air cushions are also available in “stroke to open,” “stroke to close” or both directions, whenever needed.

The air cushion consists of a manifold assembled between the top of the cylinder and the end cap (if deceleration is desired in the “stroke open” direction) or between the yoke and the bottom of the cylinder (if deceleration is required in the “stroke close” direction).

Air cushions work by restricting the exit of air from the actuator at the end of the actuator stroke. This happens when the cushion spear engages into the cushion seal. The trapped air is compressed by the inertia of the load; the rate at which it exhausts is controlled by the actuator’s cushion needle. By limiting the exhaust, the pressure in the top of the cylinder increases and slowly decelerates the piston.

Evaluating air cushion requirements

Air cushions are not always needed. For large valves (8 nominal pipe size and larger), to estimate air cushion requirements based on velocity at impact, the formula in Eq. 1 can be used:

      Vi = 3 (Stroke/Timesec)              (1)

where:
      Vi = Velocity at impact (cm or in./sec)
      Stroke = Valve stroke (in cm or in./sec)
      Timesec = Required stroke time (in sec)

The factor of three in the equation is to take into account the time to accelerate from being at rest.

This guideline suggests that an air cushion should be considered when using an actuator with a velocity at impact of 60.96 cm/sec (24 in./sec) and is strongly recommended when the velocity at impact is 91.44 cm/sec (36 in./sec).

Additional considerations

A variety of ASVs are available on the market. Aside from considering the previously mentioned capabilities, which have a direct impact on ASV reliability and performance, the following abilities deliver additional advantages, as outlined below.

Ease of setup, tuning and maintenance: In addition to being durable, ASVs should be easy to maintain. Consider valves and actuator systems that can be easily disassembled and reassembled in the field. Onsite technicians’ abilities to repair and tune the valve without relying on factory experts lowers operating costs, reduces critical downtime and minimizes maintenance workloads.

Noise and vibration control: Over time, excess noise and vibration loosen system components, resulting in additional maintenance labor and costs. Consider valves that operate within the 85 dBa–95 dBa range. Certain ASVs use control valve trims to reduce noise (and vibration) by up to 30 dBa through staging, frequency shifting, attenuation and velocity control.

Extensive OEM testing: Consider valves that have undergone extensive OEM testing and evaluation. OEMs that use testing methods such as computational fluid dynamics (CFD) analysis can ensure that valves will perform as expected under a variety of conditions. CFD is used to analyze, optimize and verify the performance of valves to confirm that they will perform as initially designed.

Recommendations

ASVs play a vital role in compressor safety and plant performance. By installing ASVs, facilities can run compressors up to the surge line, thereby increasing productivity without worrying about surge events. While a variety of different ASV designs are available, specific features—such as four-way actuation, air cushions and positioner feedback mechanisms—make for a more reliable and efficient valve. HP

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