Bypass Method For Recip Compressor Capacity Control

By Ali Ghanbariannaeeni and Ghazalehsadat Ghazanfarihashemi, Tehran, Iran | June 2012 Vol. 239 No. 6

Figure 1: Schematic of compressor bypass line.

This article considers the bypass method for capacity control in reciprocating compressors. The method is the oldest in controlling outlet capacity, which is used in all types of compressors for turndown capacity between 100% and zero.

The reciprocating compressor is a positive displacement machine. During normal operation, it takes in a quantity of gas from its suction line and compresses the gas as required to move it through its discharge line.

Unlike centrifugal compressors, reciprocating compressors cannot self-regulate their capacity against a given discharge pressure. In fact, it keeps compressing without limitation unless we specify the range of work, and that capacity will be a unique quantity at any point in time. Thus, we have a real need to control the capacity of the reciprocating compressor.

Moreover, in most instances, a reciprocating compressor needs to be unloaded by a bypass line for startup. Otherwise, if the compressor is started in full-load conditions, the driver’s required torque during startup will be 350% of nominal torque. By comparison, electrical motors are designed for 40-60% of starting torque capability. This is another reason for using a bypass line. (1,2)

Bypass Method
Capacity control by means of an overall bypass can be applied without limitations to all compressors; it provides the recycled gas entering the suction line close to the normal suction temperature. This arrangement is normally employed for starting and shutdown purposes. It covers zero to 100% range as well.

When capacity turndown is required, a bypass with a control valve is necessary. Depending on the system requirements, the bypass may only be across the first stage. More often, the bypass spills back across all stages unless the differential pressure is too high to be handled by a single control valve. Maximum allowable pressure drop through control valve shall be specified by the compressor manufacturer.

The take-off point for the bypass must be at the downstream of a heat exchanger so that cooled gas will be spilled back to the suction. If there is no exchanger in the discharge, the bypass must branch into the suction line upstream of an exchanger. As an alternative, a cooler may be placed in the bypass line. This means that an after-cooler or a bypass cooler may have to be used. In addition, a check valve in the discharge pipe is required to prevent the high-pressure gas from flowing back when the compressor is at standstill. In any case, the bypass should tie in upstream of a suction knockout drum so that any condensate resulting from the expansion cannot enter the compressor (Figure 1).

Bypass control is preferred compared to other methods because of its smoothness, simplicity, low initial costs, high practicality and ease of application. Because this regulation method is very uneconomical, it should only be used if the compressor has to be operated at reduced capacity for a short time or in combination with another energy-saving type of capacity control such as unloader valve, clearance pocket and step less method.

When a bypass is used in combination with step control, five-step operation is more efficient than three-step operation.