The efficiency and availability of a plant’s compressors have a direct impact on the profitability of a liquified natural gas (LNG) facility. Downtime can affect the plant’s mixed refrigerant loops and lead to reduced LNG production, along with substantial contractual penalties. One cause of unexpected downtime is often poorly performing anti-surge valves.
Joe DeMonte, Global Midstream Oil & Gas/LNG Industry Manager for Emerson, says compressors represent some of the most valuable pieces of equipment within an LNG facility and it is the job of the anti-surge valve to protect them. His article, Optimising Performance in the August 2018 issue of LNG Industry magazine, discusses how to achieve and maintain the highest level of performance from anti-surge valves.
Joe points out that an anti-surge valve provides recycle flow to each stage of multi-stage compression trains commonly used in LNG liquefaction processes.
The primary purpose of the anti-surge valve is to protect the most critical and expensive pieces of equipment in many LNG facilities—the compressors. During startup, the anti-surge valve provides throttling control to recycle a portion of the discharge flow as the compressor is brought up to capacity.
To prevent surge, the compressor’s automation system modulates the anti-surge valve for throttling control. Joe says this requires a fast, stable response to the automation system’s open-loop step command. During a surge event, the valve must open quickly—in less than two seconds—to recycle the discharge gas back to the suction side of the compressor.
If the valve fails to react quickly, severe damage to the compressor can occur. In a potential compressor trip scenario, a signal will be sent to a solenoid that commands the anti-surge valve to open in less than one second.
Many factors come into play when selecting an anti-surge control valve, but two are very important: reducing noise and vibration. A typical customer requirement for control valves is to keep this noise below 85dBA for any extended period of time. Joe asks if this is necessary:
The 85dBA limit is a requirement for continuous service, but the anti-surge valve spends most of its life closed. When it operates, it does so to catch a surge event, and then returns to the closed position. For these and other reasons, it’s a good idea to look closely at noise requirements. Is 85dBA really necessary? Can the noise limit above normal operating conditions be relaxed?
As LNG plants scale up, there typically is less room to spread out. Instead, the LNG facilities get built on floating vessels, or on small patches of land near shore. This leads to shorter piping runs, multiple elbows and thinner schedule piping needed to fit into the smaller space, leading to excess vibration at the anti-surge valve. Positioners, accessories and even tubing exposed to such vibration can fatigue and fail in this environment. Joe offers advice:
One way to improve the reliability of an anti-surge valve is to use a positioner with a non-contact valve position feedback. This eliminates potential early failure due to part wear. Spool valve positioners can plug and should be avoided. Volume boosters that use an angle-body type design can be flange-mounted to the actuator, helping to prevent vibration-related issues. Flex hose should also be avoided to improve reliability. Accessory tubing should be checked to ensure the natural frequency of the tubing far exceeds that of the potential vibration levels in the field.
Whereas 10 years ago there was a push toward the “Mega” train concept, there is now a combination of traditional LNG capacity, along with smaller “Mid-Scale” LNG trains, new licensors and smaller scale floating LNG. Even within a single plant, anti-surge valves can range in size from NPS 4 to NPS 48. A site with a variety of different sized valves needs to be able to take advantage of common parts.
An ideal anti-surge package uses the same positioners, trip valves, solenoids and boosters on all sizes of valves. The only thing that should vary between the different anti-surge valves should be valve size and actuator size. As the valve gets bigger the actuator must get bigger, but that shouldn’t mean the positioner or accessories should change.
If an issue arises, the plant needs to know how to interact, troubleshoot, tune and commission its anti-surge valve to get up and running again. Start-up time should also be a quick process without having to retune boosters, positioners, controllers, etc.
The valve should have a user-friendly interface for tuning and set up. One way to achieve this level of familiarity is to use a common positioner across the plant; i.e., use the same positioner on anti-surge, general service, critical service and SIS applications.
Joe suggests obtaining a specialized anti surge valve package:
Emerson’s Fisher Optimized Digital Valve Package (ODV) takes into account all of the key anti-surge requirements to provide a high performing and reliable product for LNG facilities. The ODV package combines Fisher valves, advanced noise abatement Whisper Trim technology, boosters and FIELDVUE positioners for anti-surge applications.
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