Motor Driven Compressor Suction Valves

1. Purpose

This procedure provides reasons for installing, and failure action of, a throttling valve located in the suction line of a motor driven centrifugal compressor in recycle gas service.

2. General

A number of processes require the use of centrifugal compressors in recycle gas service. It is important to design the system to prevent compressor damage caused by operating conditions that produce either too low (surge) or too high (choked) a flow through the compressor. It is also desirable to throttle the suction valve to obtain a proper H2/Hydrocarbon ratio during long term turndown scenarios which saves utilities and allows a more desirable coke laydown on the catalyst.

This procedure deals primarily with motor driven compressors, however, some mention is made regarding turbine driven machines.

3. Motor Drives

If a motor drive is selected for a centrifugal compressor, the following shall apply:

3.1 Compressor Operation

Figure 1 shows an example of a motor driven centrifugal compressor in a catalytic reactor circuit. The motor driven compressor operates at a fixed speed with a single operating curve. The design point on the curve represents the process conditions for design and the curve provides a flow/head relationship at any operating point (see Figure 2). The surge line theoretically defines the absolute minimum actual cubic feet per minute (acfm) for safe operation. Compressor operation at or less than the surge line acfm, even momentarily, may result in mechanical damage to the compressor internals.

Compressor operation at an acfm considerably larger than the design (in the “choked flow” region) should also be avoided. This situation will also cause increased stresses on the compressor internals.

Since the compressor circuit is generally not designed to control the gas flow rate, the acfm of a motor-driven centrifugal compressor will change with changes in system hydraulics. If the liquid feed rate to the system is decreased, the system P will decrease and the compressor acfm will increase to a point on the curve where the acfm and the new head are in equilibrium. If the system P change is large (which occurs at reduced feed rate operation), the acfm will increase into the choked flow region unless an artificial P is created that minimizes the overall head change of the compressor.

3.2 Compressor Suction Valve

A control valve, with a lockable mechanical limit stop, is installed in the compressor suction line to create an artificial P for the compressor and prevent long-term operation in the choked flow region. This control valve will:

a. Reduce the motor power consumption and limit the increase in acfm, during operation at reduced feed rates, and obtain an optimum H2/Hydrocarbon ratio.

b. Maintain compressor operation in a stable region during a total instrument air failure.

All pneumatically operated control valves with spring diaphragms have a fail action. The fail action of each control valve in a process is selected to provide safe operation of the process and mechanical equipment during a refinery wide total instrument air failure.

The spring action of the suction valve that provides the maximum protection for the process and equipment for a motor-driven centrifugal compressor, is “FAIL CLOSED”. The actual spring action is “fail closed to minimum stop” because the travel is limited to the mechanical limit stop position. The fail closed to minimum stop action of the compressor suction valve provides safe operation of the compressor during an instrument air failure by preventing an increase in acfm, thus preventing compressor operation in the choked flow region.

It is intended to install the valve downstream of the suction strainer. When throttling, the additional acfm will generate more P, therefore, minimize piping between the valve and compressor inlet.

4. Turbine Driven Recycle Compressor

Turbine-driven centrifugal compressors normally do not require a control valve in the suction line for equipment protection because the compressor speed is variable. The equilibrium operating point of a variable speed centrifugal compressor will change between an infinite number of operating curves for different speeds, and this will limit the magnitude of the variation in acfm for changes in system P.

If a suction valve is required for a turbine-driven centrifugal compressor for process reasons, the valve design would be exactly the same as for a motor-driven compressor, i.e., fail closed to minimum stop, and without any accessories such as a pneumatic lock-in-place relay. This action ensures that compressor operation will be in a stable region during an instrument air failure.