SURGE TEST OF A CENTRIFUGAL COMPRESSOR – SINGLE STAGE

The basics of compressor surge have been already discussed in earlier posts. In this post, the procedure regarding surge test will be discussed. The basic line diagram consisting of suction volume bottle / suction scrubber, compressor, anti surge valve, discharge line NRV & discharge isolation valve is shown in the following figure.

 

Preliminary activities to be done before start up of compressor and conducting surge test in site are as follows.

1.       Inspection of suction piping – No material shall present inside the suction spool. Chance of material to be placed before suction filters installation and assembling of suction piping to the compressor suction.

2.       Inspection of suction strainer and inlet filters – Suction strainer shall be cleaned and installed in position. Inlet filters shall be installed as per the manufacturer’s recommendation.

3.       Functional checking of IGV (Inlet Guide vanes) – IGV functional checking is carried out by opening and closing in offline mode (manual operation) and online mode or auto mode (remote operation).

4.       Functional checking of ASV (Anti Surge Valve) – ASV functional checking shall be carried out as per manufacturer’s recommendation or procedure and calibrated ASV shall be used.

5.       Setting of probes – Instrumentation setting of radial and axial probes shall be done to record proper vibration readings during surge.

6.       Calibration of gauges and transmitters – During surge test of centrifugal compressor, the surge line and safe operating region is determined by the parameters of flow and pressure across rpm of the compressor. To avoid errors, calibration shall be done to all the gauges and transmitters involved in it.

7.       Inspection of gearbox (if installed) – Visual inspection of gearbox shall be carried out and to be ensured that no corrosion is present on the gear and pinion. Manual barring can be carried out to check the working function of the gearbox. If requires servicing to be done for the gear box.

8.       Lube oil flushing – Lube oil flushing shall be done for all the lube oil piping and auxiliaries installed (lube oil coolers, filters, lube oil tank & lube oil tank strainer). Lube oil flushing shall be done until DP across the filters reaches to the normal operating range.

If required lube oil flushing can be carried out in two phases like lube oil 1^st flushing and lube oil 2^nd flushing. Difference between two phases of flushing is bypassing some of the auxiliaries in the lube oil system.

9.       DOR (Direction of Rotation) of motor – DOR of compressor motor shall be ensured as per operating manual. Correct if DOR is reverse.

10.   DOR of AOP (Auxiliary Oil Pump) –DOR of AOP motor shall be ensured as per operating manual. Correct if DOR is reverse.

 

SURGE TEST

Surge test shall be performed as per the vendor recommendation and as per the procedure mentioned in the OEM manual.

The generalised procedure for surge test is as follows:

a)      Close the discharge isolation valve of the compressor.

b)      Open the ASV up to 100% and make the arrangement to vary the position of ASV.

c)       Record discharge pressure, discharge flow in DCS at 100% opening of ASV and at fixed opening position of IGV.

d)      Slowly close the ASV i.e.do decrease the opening percentage of the ASV.

e)      During closing of ASV the discharge flow will be observed in decreasing pattern.

f)        At certain opening percentage of ASV, sudden drop will be observed in discharge flow and also abnormal noise will be observed.

g)      Now, record the discharge pressure and discharge flow values at this point.

h)      After recording the discharge pressure and discharge flow values, immediately open the ASV to 100 percent to shift the operating point of the compressor away from the surge limit.

i)        Repeat the above steps for various opening positions of IGV.

j)        From the obtained discharge pressure and discharge flow values at surge points, calculate the gain and bias values of the surge control system and incorporate in DCS system.

k)      Based upon the calculated gain and bias values determine the surge parameter of the operating point. (Surge parameter is the margin available between operating point and surge point).

The surge parameters calculation and incorporating in the system & plotting of surge line will be discussed later.