Multistage Recip Compressor Pressure Control Schemes

1. Purpose

This procedure explains the operation of pressure control systems specified for multistage reciprocating compressors.

2. General

Multistage reciprocating compressors are commonly used for net gas compression on Platforming Process units and for makeup gas systems on hydrogen processing units, e.g., Unionfining and Unicracking Process units.

The pressure control systems for multistage reciprocating compressors serve two primary functions: they control plant pressure and protect compressors during upsets. Reciprocating compressors are constant volume machines. In order to control the net amount of gas pumped by these machines, some of gas is spilled from the discharge of the machine back to the suction. If the suction pressure of a given stage falls, or the discharge pressure rises, the compression ratio may increase to a point where the rod loading exceeds the design, causing a failure, or discharge temperatures exceeding design limits are reached. By controlling the suction pressure for each stage, the compression ratio across each stage of the machine may be maintained and such problems may be prevented.

There may be other variations of plant pressure control schemes that involve multistage reciprocating compressors; however, the two examples included in this procedure should provide the basic knowledge needed to understand such control systems.

3. Schematic Diagram Symbols

This procedure includes schematic drawings of the instrumentation used to control a two stage Platforming net gas compressor and the Unicracking Process unit makeup gas compressor. The symbols used in the schematic diagrams are the same as the symbols used on the Piping & Instrument Diagrams (P&IDs) for this service. Each symbol’s function is briefly described below:

3.1 Pressure Controller

A direct acting controller increases its output signal if the process variable increases. A reverse acting controller decreases its output if the process variable increases.

3.2 Multiplier

Output equals the input signal multiplied by a constant. A multiplier may also invert and/or bias signal relationships.

3.3 Low Signal Selector

Output is the lowest of two or more inputs. When associated with a Fail Open control valve, the valve opens as the signal decreases.

4. Net Gas Scheme

Pressure controllers are located on the Platforming unit Separator and the suction drums for each compressor stage. These controllers operate the compressor spillback valves and the net gas control valves through a series of signal multipliers and low signal selectors. Either of two pressure controllers can control the compressor spillback valves. See Figures 1 and 3.

4.1 Normal Operation

This system controls the pressures of all three systems (Separator, Recontact Drum No. 1, and Recontact Drum No. 2) as the net gas flow changes. The Separator pressure controller, PIC-04, controls first stage spillback valves PV-04A and B through multiplier PY-04H and low signal selector PY-04G. This controller also operates vent valve PV-04C. PIC-20, the pressure controller for Recontact Drum No. 1, controls second stage spillback valves PV-20A and B through multiplier PY-20F and low signal selector PY-20E. PIC-21, the pressure controller for Recontact Drum No. 2, controls net gas control valve PV-21 through multiplier PY-21B.

4.2 Pressure Rises

If pressure in the Separator rises, more material must be removed from the system in order to return the pressure to the setpoint. PIC-04 will first close the first stage spillback control valves PV-04A and B, thus causing more gas to go forward from the Separator. As the first stage spillback valves close, the pressure in Recontact Drum No. 1 will tend to rise above the setpoint of its controller. PIC-20, the pressure controller on this drum, will close second stage spillback valves PV-20A and B, causing more gas to go forward from Recontact Drum No. 1. This in turn causes the pressure in Recontact Drum No. 2 to rise. An increase in this pressure causes pressure controller PIC-21 to open net gas control valve PV-21, thereby increasing the flow of gas from the unit.

If the system outside the unit will not accept any additional gas flow, then control valve PV-21 will reach the fully open position and the pressure in Recontact Drum No. 2 will continue to increase. The controller output signal from PIC-21 will eventually exceed 50% and be routed to low signal selector PY-20E via the inverting multiplier PY-21C. PIC-21 will then take over operation of PV-20A and B valves, opening them to release gas from Recontact Drum No. 2 back into Recontact Drum No. 1.

This will cause a pressure increase in Recontact Drum No. 1, so the output of controller PIC-20 will exceed 50%. This signal is sent through inverting multiplier PY-20G to low signal selector PY-04G and then takes over operation of the PV-04A and B control valves, opening them to release the gas from Recontact Drum No. 1 back into the Separator. The Separator pressure will then increase, causing the output from PIC-04 to rise above 50%, which then opens control valve PV-04C, to vent gas to the relief header. This will dispose of the excess gas while maintaining control of all three vessels. Thus, the flow of liquid leaving these vessels is not significantly disturbed and any intermediate gas streams continue to operate at the same pressure.

4.3

If pressure in the Separator falls, output from PIC-04 will fall and cause spillback valves PV-04A and B to open more. The pressure controller for Recontact Drum No. 1 will maintain the setpoint by opening the second stage spillback valves. The pressure controller for Recontact Drum No. 2 will close the net gas control valve more, thus reducing the amount of gas leaving the unit. In a major Platforming unit upset, such as a heater or recycle gas compressor shutdown, the plant pressure control scheme may maintain Separator pressure by putting the net gas compressors on total spillback.

5. Makeup Gas Scheme

This control scheme comes from a Unicracking Process unit makeup gas compressor. Like the Platforming unit net gas compressor example, there is a pressure controller on the Separator and on each of the compressor suction drums. This system protects the compressors and maintains control of the High Pressure Separator and the interstage pressures as the demand for gas varies. See Figures 2 and 4.

Some Hydroprocessing Units may be designed without suction drums for the Makeup Gas Compressor. In this case, the Makeup Gas Compressor spillback flow may be controlled by a single spillback on the final stage discharge, spilling back to the first stage suction. The normal and regeneration modes of operation described in the following sections are still applicable for these schemes.

5.1 Normal Operation

The pressure in the High Pressure Separator is regulated by varying the amount of makeup gas brought into the unit through control of the third stage spillback valves. The makeup gas rate is the amount of gas required to maintain plant pressure, which is also the minimum makeup gas requirement.

The third stage suction drum pressure controller normally regulates the second stage spillback valves; however, if the third stage suction pressure decreases, it will take over control of the third stage spillback valves and open them. The second stage suction drum pressure controller normally regulates the first stage spillback valves; however, if the second stage suction pressure decreases, it will take over control of the second stage spillback valves and open them.

The first stage suction drum has one pressure transmitter but usually has two pressure controllers (A and B). PIC-27A has a set point lower than the normal suction pressure, but high enough to protect the machine from a high compression ratio. The output of PIC-27A opens the first stage spillback valves via a low signal selector if the first stage suction pressure goes too low.

The other first stage suction drum pressure controller, PIC-27B, controls the vent valves associated with that drum. PIC-27B output is usually split ranged and upon high pressure first opens a vent valve to fuel gas, and then opens a vent valve to the relief header. A vent is required because during certain shutdown scenarios, the last stage spillback opens, and may spill back so much gas to the first stage suction drum that the drum pressure could rise and cause the PRV to lift if the gas is not vented. When this is the only purpose of PIC-27B, PIC-27B has a set point that is somewhat higher than the normal pressure of the drum. PIC-27B holds the vent valves closed in normal operation, but opens them when necessary.

Another reason for the first stage suction drum vent in some units is that the source of gas supplies more gas than the unit consumes. In this case PIC-27B normally vents the excess gas, and the set point of PIC-27B equals the normal operating pressure.

Before 2004, when the first stage suction drum had vent valves, Inflection Point Engineering used a single PIC split ranged to three destinations, namely, the spillback valve and the two vent valves. That system had problems related to tuning and difficulty selecting a set point for correct operation of both the spillback and vent valves. It was replaced by the two-PIC system in 2004 to resolve those problems.

If a higher purity recycle gas is desired, the operator sets FIC-31 at a rate, typically about 10% of recycle gas flowrate, which will increase the makeup gas rate and improve the recycle gas purity. The High Pressure Separator controller overrides the FIC controller via the high signal selector in the event of high pressure in the separator (which occurs mainly at start-up when feed is first introduced into the unit).

5.2 Regeneration

The regeneration mode of operation is another feature of this makeup gas scheme. During catalyst regeneration, FIC-30 controls the flow of plant air to the unit through switch FY-30. The plant air for regeneration flow controller regulates the net amount of air pumped into the unit via the third stage spillback valves. The first, second, and third stage pressure controllers function the same as they do in mode 1. The pressure of the High Pressure Separator is regulated by venting regeneration waste gas to atmosphere.

Figure 4