Specifying Trip Valves is Critical for LNG Service
LNG equipment overview. The process of liquefying natural gas has been in existence since the 19th century. By converting natural gas from a gaseous state into a liquid, it can more easily be transported to areas where natural gas pipelines do not reach.
According to the U.S. Energy Information Administration, the volume of natural gas in a liquid state is roughly 600 times smaller than its volume in a gaseous state in a natural gas pipeline. Some geographic areas of the U.S. store LNG onsite where there is a high demand for electricity at certain times. This demand typically revolves around extreme weather conditions or where pipeline delivery is limited. Natural gas also can be used in a liquid state as transportation fuel.
With such an essential end product, it stands to reason that the process for making it has been adequately refined over the years. The liquefaction of natural gas is achieved by lowering the temperature of the gas until it changes phase from vapor to liquid. The resulting LNG exists at a temperature of roughly –260°F. The process to take natural gas to this temperature is most often carried out in a series of turbocompressors. These turbocompressors increase the pressure and compress the gas, followed by an adiabatic expansion step that causes a dramatic reduction in temperature. The process is straightforward, but if certain precautions are not taken, disaster can ensue. One of these precautions is the inclusion of a trip valve.
Turbine trip valves. A turbine trip valve (FIG. 1) should be present near each turbocompressor used in the natural gas liquefaction process. If the load on the turbocompressor stops for any number of reasons, then the trip valve is designed to completely cut off the flow. This prevents the turbine from over-revving and potentially damaging the equipment.
The turbine trip valve is an essential safety device that not only prevents catastrophic damage to extremely expensive capital equipment, but also protects the lives of those working near the turbocompressors. A turbine trip valve is unique among large valves due to the combination of the extreme temperature conditions it faces and the short time allowed for a turbine trip valve to cycle closed. In fact, it is common for trip valves to be required to cycle closed in 0.3 sec–2 sec, which can be difficult to achieve for large valves.
To properly specify these trip valves, a number of different factors must be considered. The first step is to understand the application requirements. These include the pressure, temperature, flows and media that are making their way through the valve.
Specification requirements. Some of the steps in properly specifying a turbine trip valve are similar to those taken in specifying any type of valve. The first is to determine how much fluid will be flowing through the valve. The actual flowrate will determine the size of the valve required for the job. In most cases, the required size will make a high-performance butterfly valve the best choice for this application.
Next, the typically low process temperatures must be taken into account. It is not unusual for natural gas trip valves to face design conditions below –100°C (–148°F). These cryogenic temperatures require thick piping insulation, and the valves need long stem extensions so that the critical stem sealing components are outside of the cold zone.
The next step is to determine the required torque, which is a function of the valve size, seat and bearing designs, as well as process temperatures and pressures. An analysis of the required torque and cycle speed will allow the calculation of the maximum rotational force that the long stem extension will need to withstand. It will also determine how much “shaft windup” will result.
Once the maximum rotational force is known, an actuator must be selected to provide the required torque. An adequate safety factor should be applied to accommodate process upsets, aging equipment, etc. The extremely fast required cycle times frequently make oversized air ports necessary.
Once the major components—the valve and actuator—have been selected, similar care must be used to select the control components. Similar to the computer modeling used to size and select a process valve, the pneumatic system also must be modeled to allow the selection of solenoid valves and quick exhaust devices to vent the air from the actuator quickly enough to meet the trip requirements. All sources of pressure loss must be accounted for, such as pipe nipples, compression fittings, elbows, tees and exhaust screens.
Other considerations. A few other important characteristics, if not recognized and addressed properly, can be problematic. For example, the combination of the mass of the moving components of the valve and actuator and the fast cycle speeds can cause the actuator to be damaged. In this case, shock-absorbing bumpers must be specified for the actuator travel stops.
The locality where the trip valve will be installed must be accounted for. All components must meet local piping codes and electrical certifications, and the ambient conditions may require sunshades, corrosion-resistant materials or coatings, air system “rebreathers” or heat tracing.
Testing. Finally, each trip valve assembly must be tested to verify that it meets all the system requirements. Most customers require certified material test reports to confirm that the critical pressure-retaining components are made from the alloys specified by the manufacturer. Other forms of nondestructive examination (e.g., X-rays, liquid penetrant tests, positive material inspection, etc.) also may be required by the customer.
After the trip valve system has been completely assembled, it must be tested and timed to verify that it meets the trip speed requirement. This step cannot be skipped, as computer modeling alone is not sufficient for this critical piece of safety equipment. This testing is typically done using data loggers and sensors. The sensors detect valve position and air pressure, allowing the test system to graph the valve through the complete cycle.
Heavy engineering is required to ensure that a trip valve will work properly and do the job needed. Understanding all of the conditions and requirements will ensure that a critical element is not overlooked that may lead to damaged equipment, unplanned shutdowns and potential injuries or deaths.
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