- Published on Monday, 16 December 2013 13:43
- Written by Loyd Hilliard
The relatively recent rise in demand for liquefied natural gas, or LNG, has fueled a boom with regards to the construction of import and export terminals. Since the early 1990’s the number of LNG exporting/importing countries has increased from 16 to 43. Couple this with the fact that large natural gas deposits are rarely located next to the huge arenas that consume them and you have the perfect storm that is the current LNG market.
Current LNG Processes
The LNG process liquefies natural gas, increasing its density by about 600 times in order to cost -effectively store and transport the product to market. There are two dominant liquefaction processes in existence at this time; the Mixed Refrigerant Liquefaction Process and the Cascade Liquefaction Process. The overall processes are similar in purpose with the main difference being the design of the main heat exchanger unit(s).
Both LNG liquefaction processes contain the following six critical control valve applications:
- inlet feed gas control valve
- rich amine letdown control valve
- amine pump recycle control valve
- gas to flare control valve
- compressor anti-surge control valve and
- Joule-Thomson/expander bypass control valve
Inlet Feed Gas Control Valve
This valve essentially regulates the inlet gas pressure to the entire liquefaction process and, as a result, reliability is absolutely critical. Sometimes known as the feed gas pressure control valve, this valve is subject to high flow rates, high operating pressures and high fluid densities. The large flow rates and pressure drops require the use of high capacity bodies with noise attenuation trim. Since the feed gas is often sour at this point, NACE material considerations are often imposed. Typically implemented with 100% operational redundancy, the use of smart instrumentation with on-line diagnostic capabilities is critical.
Rich Amine Letdown Control Valve
Designed to regulate the amine solution level in the acid gas absorber unit, the rich amine letdown control valve is subjected to a flow phenomenon commonly referred to as outgassing. Outgassing is often mistakenly modeled as “flashing”, which results in grossly oversized valves that exhibit poor control characteristics. The presence of solids or scale may also require the use of “dirty service” trim designs. Trim selection in an outgassing application such as this is dependent on pressure drop and the actual amount of entrained gas or outgassing potential. True, discrete staged trim sets utilizing axial staging coupled with secondary sections designed to reduce fluid jet diameters are most effective.
Amine Pump Recycle Control Valve
This valve serves three main functions:
- Facilitate start-up and commissioning of acid absorber unit
- Boost suction pressure of pump to ensure that it is above amine vapor pressure
- Protect pump from cavitation damage
Usually subjected to high dP/P1 ratios, this valve is typically supplied with discrete, radially staged, anti-cavitation trim designed to eliminate cavitation and associated damage to valve and pump. It is advisable to utilize a manufacturer capable of custom cage designs to help provide a trim set with a broad rangeability.
Gas to Flare Control Valve
With service conditions characterized by high pressure drops and flow rates, this valve’s primary purpose is to safeguard the process against overpressure.
Although this application is considered intermittent, the associated noise levels dictate the use of aerodynamic noise attenuation trims. It is extremely important to perform a comprehensive noise evaluation that looks at both trim and expander noise sources per the current revision of the IEC standard. If the noise evaluation does not include an analysis of both trim and expander noise sources, it is not only in violation of the IEC noise standard but also increases the risk of installing a problematic valve construction.
Compressor Anti-Surge Valve
Arguably one of the most critical valves in the plant, the compressor anti-surge control valves protect the centrifugal compressors from a phenomenon known as surge. This is one of the few valves in an LNG process where instrumentation selection is more important than the valve body selection.
The best valve body solution is a large angle construction outfitted with slotted or drilled-hole noise attenuation trim. Although it is common to impose an 85 dB(A) sound pressure level limit, this requirement is not critical as the valve operation is extremely intermittent – or at least it should be.
A frequently operating anti-surge control valve is typically indicative of an uncontrolled process and requires a system-wide evaluation to correct. It is important to recognize that both speed and throttling abilities are essential for this construction to provide proper control during startup, normal operation, and trip (surge) situations.
With regards to instrumentation, the performance criteria needed for reliable surge control will specify metrics such as stroking speed, opening dead time, allowable overshoot, control signal response characteristics, linearity, and supply droop. All of these criteria need to be considered to insure proper selection of valve instrumentation including actuator, positioner, volume booster(s), solenoid valve, trip valve, air pressure regulator, dump valve, air filter, and even tubing and mounting hardware.
The criticality of this valve necessitates conducting a factory acceptance test (FAT) to verify the performance characteristics of the fully assembled valve unit. Because of the inherent complexity of the instrumentation package, it is also advisable to have some type of field acceptance test to confirm that the integrity of the unit was maintained during shipping.
Joule-Thomson / Expander Bypass Valve
The heart of all LNG liquefaction processes is the main heat exchanger and the Joule-Thomson (a.k.a. expander bypass) valve is tasked with controlling the refrigerant used in this unit. In addition to withstanding fully cryogenic temperatures, these valves must be able to attenuate aerodynamic noise, handle fluids with entrained gasses and provide tight shutoff (TSO). Normally, the ability to offer a TSO construction is fairly easy, and inexpensive, but when coupled with cryogenic temperatures, it becomes particularly demanding.
The first issue is the absence of a cryogenic seat leakage standard applicable to control valves. A standard is currently being developed by ISA, but is a few years away from publication. As a result, users have been forced to adopt their own requirements or even apply shutoff benchmarks that were designed for isolation valves. One of the biggest challenges is determining how to obtain the cryogenic temperatures within the valve during the test.
A liquid nitrogen bath cools the valve from the outside, but the actual process fluid flowing through the valve cools from the inside. In order to make a simple “dunk and measure” test meaningful, the cooling rates must be closely controlled to ensure the delta between valve body temperature and trim temperature eliminates thermal expansion issues within the assembly.
Early understanding of technical challenges and proven solutions improves the mitigation of project schedule risk and potential construction or commissioning delays. Providing effective and reliable protection of critical and costly equipment is of the utmost importance. Control valve failure or sub-optimal performance can shut down a complete LNG train, and with gross revenues ranging from $16 million to $20 million per day, that simply isn’t an option.
All images courtesy of Emerson Process Management