Module 10

How to do compressor performance test at site

The Following Testing Procedure Should Be Strictly Followed In The Same Order Of The Given Steps.

It Should Be Carried Our By Experienced Engineers Who Have Good Knowledge in both Instrument & Control and Process Engineering.

Any Deviation from the Above May Endanger the Compressor

Figure 10-1 shows the P&I’D for the compressor that will be tested.

The subject compressor is driven by a gas turbine and therefore its speed is variable.

The following are the steps that should be followed for performing the test, once more these steps should be followed in the same order.

Figure 10-1Step (1)

(a) Prepare a table similar to the one shown in table 10-1 which includes:

Speed: as % of the designed operating speed as shown in the original vendor’s performance curve

Flow: Inlet or outlet (SCFH) depending on the location of the flow element and the original vendor’s performance curve

Suction Pressure P1: in PSIG

Discharge Pressure: in PSIG

Suction Temperature: in °F

Discharge Temperature in °F

Antisurge valve position: %

(b) From the original vendor’s performance curve, determine the surge flow at the given speeds (e.g. 100%, 90%, 80% etc.)

Convert these flows into standard units using the equation 10-1 & 10-2 given later in this module.

(c) Arrange with the plant laboratory to take a sample of the gas at the compressor suction, and find the molecular weight.

Table 9-1

SpeedInRPM

Q(UNC)SCFH

P1PSIG

T1°F

P2PSIG

T2°F

FV%

Step (2)

(a) Make sure that the compressor is running in its normal conditions (speed, pressure, flow, .. etc.) i.e. there is no plant upset or any abnormal conditions.

(b) Arrange to have one field operator standing beside the compressor in the field, and has a radio to report any abnormality to the control room or to the people who are performing the test.

(c) Switch the antisurge controller to MANUAL mode. Gradually open the antisurge valve manually until it is fully open

(d) Check the running speed and adjust the turbine governor to get the speed as close as possible to the required speed (e.g. 100%, 90%,.. etc.)

(e) Leave the process to be stabilise at these conditions for 5—10 minutes.

(f) Take the first set of readings and record it in the table.

Step (3)

(a) Gradually, close the antisurge valve manually by 5-10 %

(b) Check the compressor speed and if necessary adjust the turbine governor to give the same compressor speed adjusted in step (2.d).

(c) Leave the process to stabilise for 3-5 minutes.

(d) Take the second set of readings and record it in the table.

STEP (4)

(a) Repeat the same procedures listed in step (3) and record the readings in the table.

(b) When the flow is approaching the surge point which was determined in step (l.b) above for this particular speed

OR

When you see that the flow is dropping very fast and sharp, you have to be careful as this means that the compressor is close to the surging point.

(c) Repeat the same procedure listed in step (3) but with only 1% to 2% antisurge valve closure instead of 5%, until you reach the surge point which will be seen as:

1. Very sharp drop in flow followed by sharp increase.2. Very sharp increase in the discharge pressure followed by a sharp drop.3. Big noise in the compressor (to be confirmed by the field operator).

At this moment, open the antisurge valve quickly to the full open position.

The data for this surge point should be recorded in the table and marked as the surge point.

Note that due to the need for immediate opening of the antisurge valve at the moment of surging, you may not be able to take the readings, therefore the readings could be taken from the recorders.

Step (5)

(a) Leave the process to stabilise for 5—10 minutes, with the antisurge valve fully open.

(b) Arrange to adjust the compressor speed to the next desired speed (e.g. 90%).

(c) Repeat the same procedures of steps (1) through (4) and record the data accordingly.

(d) Repeat the same for a new compressor speed (e.g.80%,70%) until you collect the data for all the desired speeds.

Now, you have collected all the data which would be required to plot the new performance curve.

The only thing that you need to check now is the type of the original vendor’s performance curve. There are mainly three types:

(a) flow vs. discharge pressure (simple calculation)

(b) flow vs. pressure ratio P2/fl (simple calculation)

(c) flow vs. polytropic head (complicated calculation)

The following formulas could be used for the different types above:

Qa = Qs (corr.) x (Td/520) x (14.7/Pd) Equation 10-1

Qs (corr.) = Qs (uncorr.) x Equation 10-2

n = log (P2/P1) log (P2/P1 x T1/T2) Equation 10-3

Hpol = (n/n-1) x (Z1RT1/M.g) (P2/P1) (n-1)/n -1) Equation 10-4

Where

Qs (uncorr.) The flow reading on the flow recorder SCFH

Q (corr.) = The flow corrected to the flow element design (sizing) conditions SCFH.

Qa = Flow at the actual inlet conditions CFH

Hpol = Polytropic Head in meters

T = Temperature at suction R

P = Pressure at suction PSIA

H = Molecular weight

d = compressor design conditions

s = Flow element sizing conditions

o = Operating conditions

z = Compressibility factor

g = Gravitational acceleration = 9.81

R = Universal (absolute) gas constant = 8314.34

1 = Suction conditions

For our example, the vendor’s compressor performance curve is of the flow vs. polytropic head type.

From this new plot which is superimposed on the old performance carve, we can judge whether the compressor performance has bean changed from its original design or not.

Figure 10-2 compressor performance curve

Table 10-2 shows the complete data after the calculation. Now, the data are ready to be plotted on the original performance curve as shown in fig. 10.2.

Table 10-2

NRPM

QuncSCFH

P1PSIA

P2PSIA

T1oR

T2oR

QcorSCFH

QaCFH

P2/P1 T1/T2 n Hpoolfeet