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Applying Stonewall Control to a Turboexpander
BY TARIQ H. AL-ALSHAIKH & TALAL AL-RASHIDI
A turboexpander is a high-speed centrifugal rotating equipment item that chills an expanding gas and converts the gas’s pressure energy into mechanical work. The turboexpander can also drive a brake compressor or an electric generator. Turboexpanders are used in NGL Plants to liquefy all the ethane from the sales gas and recover most of the throttled poten-tial energy as work.
Gas expansion in a turboexpander produces more liquefaction due to its ability to reach a lower chilling temperature, as the process is almost isentropic. Then the gas enters a demethanizer column, which separates the methane from the other heavy hydrocarbon ends such as ethane, propane, and butane. This design is provided to recover all the propane and heavier components while achieving an optimum ethane recovery of 99+%.
The overhead product of the demethanizer is cold residue gas, which is routed to a cold box heat exchanger, before the gas is compressed by the compressor end of the expander/compressor, where the gas is boosted in pressure. It then flows to the battery limits, before feeding the sales gas compressors.
While in either ethane recovery or ethane rejection mode, the process can be run in what is known as Joule-Thompson (J-T) mode, as well through the use of a bypass line, which is usually used during startup and shutdown of the machine. In this mode, the expander/compressor is completely bypassed. Instead, vapor feed to the demethanizer is routed through a bypass valve around the expander. Due to the expander being bypassed, the compressor must also be bypassed as no power is supplied to it. Rather than flowing through the compressor to the battery limits, the stream flows through a bypass line fitted with a check valve to prevent backflow. While operating in this mode, it is possi-ble to adjust the operating conditions, such as increasing the operating pressure of the demetha-nizer column to maintain the desired product quality. However, a loss in product recovery should be expected.
Normal natural gas liquids (NGL) recovery plants use an inlet flow to control the gas being depres-surized to the demethanizer operating pressure, using the expanders or partial/full J-T bypass valve. The inlet flow is controlled by two separate flow controllers, one for the expander inlet guide vanes and one for the J-T valves. While operating parallel turboexpander-compressor trains, one train may trip out of operation due to a fault. If the plant’s production needs to be kept high while one turboexpander-compressor is down, this is usually accomplished by operating with one turboexpander-compressor and one JT-valve. The speed override controller and low select is usually included by a compressor control vendor to take control of the inlet guide vanes to prevent unnecessary overspeed trips due to flow controller action. During this event, the compressor side of the turboexpander will go to the stonewall region as the flow from the demethanizer overhead will be directed to one compressor instead of two as the other one tripped. The compressor operation in stonewall will be sustained unless back pressure is created downstream of the compressor outlet.
Note: The stonewall region is when a compressor operating point moved to the right of the per-formance curve at its minimum head in which any additional flow or system resistance reduction will not cause any flow increase. Moreover, the com-pressor will experience stonewall when the gas velocity increased until it reaches the sonic veloc-ity of Mach 1.
Applying stonewall control to compressors is rare and not as prevalent as it is for surge control. However, it is advisable for high-speed compressors to control their operation in the stonewall region. If the compressor is operated in the choke region, it generates turbulence and vibration that can excite the natural frequency of the impeller and a sudden increase in axial thrust that the magnetic bearings will not be able to counterbalance, resulting in possible damage to the compressor, thus trip-ping the unit. Therefore, it is advisable to control the compressor away from the stonewall region by applying corrective control action to move the operating point out of the choke very quickly.
An NGL plant that has two turboexpanders operat-ing in parallel, faced a reduction in production. Management decided to switch to a different oper-ating mode that used one turboexpander-compressor train operating along a JT-valve. The turboexpander tripped due to high radial vibration (unbalance). The other turboexpander was started and ran alongside the JT-valve. After running for approximately 12 hours, the second turboexpander train also tripped on high radial vibration. Neither turboexpander could run at normal operating speed, as they tripped on high unbalance vibration if the speed slightly exceeded the minimum.
Their operation was shifted back to two turbo-expander trains operating in parallel, but at minimum speed to avoid tripping. When time allowed to inspect the machines, it was found that both compressors had broken a piece of the leading edge of one of the vanes. After investigation it was concluded that both turboexpander compressors ran in the stonewall region, which caused over-loading that excited impeller natural frequency and high vibration. Subsequently, this led to fatigue failure at the leading edge of the compressor vanes causing the severe unbalance and tripping of the train. The conclusion was to modify the control system setup to keep the turboexpander-compressor away from the stonewall region.
Although some control system suppliers utilize a suction anti-choke valve at the compressor suction side, this control scheme did not work effectively. When the suction anti-choke valve tried to prevent the compressor flow from pushing it to stonewall, the suction anti-choke valve was counteracted by the demethanizer controller trying to maintain constant pressure. As a result, the compressor anti-choke valve was bypassed, and hence did not protect the compressor from operating in the stonewall region.
A proposed control modification requires a dis-charge anti-choke valve as it will provide the proper control by increasing the speed of the turboex-pander and decreasing the J-T-valve flow. Preventing the turboexpander compressor from going into the stonewall region is achieved by creat-ing back pressure on the compressor discharge that is already preset at a certain flow in the compressor control system. The rest of the flow coming out of the demethanizer overhead will be directed to the inlet of the sales gas compressors through a bypass line upstream of the suction side of the compressors. This yields to a slight increase in the demethanizer pressure, which is within its design. Moreover, the turboexpander speed is increased to its maxi-mum possible setting, to allow for the brake com-pressor to stay within its designed operating range and keep it away from the stonewall region.
Any turboexpander control strategy to avoid operation in stonewall region should include the following:
1. Monitor the brake compressor flow, pressure ratio, discharge pressure, and expander inlet pres-sure and speed.
2. Monitor the demethanizer pressure changes for possible flow upsets.
3. Replace the compressor inlet anti-choke valve with a discharge anti-choke valve, thus increasing the head required and keeping some of the load on the turboexpander. The discharge valve will allow increasing turboexpander speed while keeping the compressor away from the stonewall region.
4. Control the Q (Flow)/N (Speed) ratio to keep it over 110% of surge Q/N value to avoid oper-ating in surge, and also keep it under 95% of stonewall Q/N value to avoid operating in the stonewall region.
5. If the flow gets close to surge, open the compressor recycle valve. It is also advisable that the recycle line is equipped with a light duty cooler that can be used to eliminate any overheating if recycling continuous for an extended period. This prevents overloading of sales gas compressors.
6. If the flow reaches the stonewall region, increase the turboexpander to maximum speed (approximately 105% of rated speed) by adjusting the variable inlet guide vanes. This change limits the tendency to move into the choke region and pushes the compressor away from the stonewall region. It also maximizes the flow through the tur-boexpander and thus minimizes the flow through the J-T valve.
7. If additional flow is required, partially open the JT-valves while monitoring the flow in the compressor, this must be done gradually under close monitoring of compressor Q/N and speed. And open the flow control valve to flare on the inlet of the brake compressor, to reduce the flow to the compressor, preventing it from reaching the choke limit. Employing a discharge anti-choke valve — in the turboexpander-compressors operating in parallel — is an essential safeguard against inevitable operation in the stonewall region upon the trip of one of the turboexpanders.
■Tariq H. Al-Alshaikh is a Senior Engineering Consultant on compressors, expanders and steam turbines for Saudi Aramco. He holds an M.S. degree in fluid dynamics from the University of Southern California.
Talal Al-Rashidi is a Compressor and Steam Turbine Engineer. He holds an M.S. degree in Mechanical Engineering from Drexel University, USA. For more information, visit Aramco.com