SURGE LINE CALCULATION OF A CENTRIFUGAL COMPRESSOR – SINGLE STAGE

Surge point is a point on the compressor operating curve where a condition of minimum flow and maximum pressure exists. It is determined on operating curve at which compressor is operating at particular operating RPM of the compressor.

Surge line is the locus of all the surge points at various operating RPM of the compressor.

 

Surge limit line and surge control line

 

Surge limit line (SLL)

As mentioned above, the surge line also defined as surge limit line (SLL). If operating point in compressor map goes to the left of SLL, then surge occurs.

Surge control line (SCL)

For the safe and stable operation of the compressor, the operating margin is provided on the surge limit line and a line is plotted relative to it. This line is called surge control line (SCL).

Normally the margin is approximately 10% of the flow and it will be as per OEM design value.

Simply SCL (Surge Control line) = SLL (Surge limit line) + 10% of the flow.

The main purpose of surge control line is it adds some margin to the surge region and this margin allow the surge control system to respond and prevent compressor from surge.

Boost line or Recycle trip line (RTL)

If back pressure is too high then the operating parameter tends to cross RTL and could lead to compressor surge. So when compressor operating point reaches RTL, controller gives the larger step opening command or multiple opening steps command to the ASV to avoid surge.

This will cause high process disturbances. There are advanced control systems which uses relatively smaller steps based on the deviation before operating point reaches to SCL.

Deviation

The distance between compressor operating point and SCL is deviation or surge parameter.

Based on the value of deviation, the operating region can be determined as stable zone or nearer to surge.

Deviation	Operating region of the compressor	Risk level
Negative	Between SLL and SCL	Alert or Alarm
Zero	On SCL	Alert or Alarm
Positive	Right of SCL (Stable and safe zone)	Normal

 

Calculation of surge control line

As discussed earlier, from the surge test, we get the surge points from the surge test as (F1,P1), (F2,P2) & (F3,P3). [if we consider surge test is performed for 3 RPMs]

Gain & Bias are the two parameters to be incorporated in the surge controller system and these values are calculated from the basic line equation.

Initially these values are calculated by considering the operating point falls on the surge control line and adapting deviation as Zero.

The representation of the basic line equation Y=mX+b is shown in the following figure.

 

 Here, in this context, the basic mathematical line equation is represented as follows.

Deviation = [(Flow * Gain) + Bias – Discharge Pressure]

Where, Y = Discharge pressure

                X = Flow

                m (Slope) = Gain

                b (Y intercept) = Bias

Hence

 Deviation = mX+b-Y .......................... (Surge control line equation)

As said above, gain and bias values are calculated initially by considering deviation as Zero.

Then the above equation becomes

                0 = mX+b-Y

                m = (Y-b) / X

Gain (m) = Y axis / X axis

                = (P3-P1) / (F3-F1) or (P2-P1) / (F2-F1) or (P3-P2) / (F3-F2)

                = G (say)

Bias (b) = Y-mX

                = P – (Gain * F) [Here P & F can be taken as P1 or P2 or P3 & F1 or F2 or F3]

                = B (say)

Hence the obtained gain and bias values are incorporated in the surge control system and surge control line equation becomes as follows.

Deviation = [(Flow * G) + B – Discharge Pressure].

As per the above table and based upon the value of deviation, the zone of the compressor can be determined whether it is stable or not and surge controller acts based upon the deviation value to make the compressor in stable operating region with the help of ASV.

 

 

 

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.

 

 

 

SURGE OF A CENTRIFUGAL COMPRESSOR

 

Surge

Surge is defined as a sudden upward movement or a sudden large increase.

Compressor Surge

For compressor surge, the sudden upward movement is the increase in discharge pressure and

Compressor surge can be defined as

ü  The point at which the compressor cannot add enough energy to overcome the system resistance or back pressure.

ü  The unstable phenomenon that occurs at low discharge flow and high discharge pressure.

ü  The phenomenon of momentary flow reversal.

ü  The Surge Point represents the minimum inlet volumetric flow where a compressor can maintain performance.

Compressor surge line is the locus of all the surge points for a compressor operating on different RPMs or different IGV positions and can be shown as follows.

Here Valve Sizing line is also known as surge margin line which is calculated as the safety factor for anti surge operation and discussed in detail under the topic of "Surge Test"

                                    

 Compressor surge cycle

1.       Compressor is working at point D; decreased inlet flow shifts the operating point operating point near to surge point A.  Continued reductions in flow will eventually trigger surge at point A.  

2.       When surge occurs, the flow direction reverses to point B.

3.       The flow reversal causes the inlet and discharge pressures to start equalizing. As pressures equalize, the reversed flow rate decreases to point C.

4.       Now the compressor regains its ability to produce head and returns to its performance curve at point D.

                 

 

Surge cycle features

• Flow reverses in 20 to 50 milliseconds. (From A to B in above graph)
• Flow regains to produce normal head in 20 to 120 milliseconds. (From C to D in above graph)
• Surge cycles (A-B-C-D-A) at a rate of 0.3 s to 3 s per cycle.

         

 Surge results in

• Compressor discharge piping vibration 

•  Rise in compressor discharge temperature 

•  Damage to seals, bearings, impeller and shaft. 

•  "Whooshing noise or "Clanking" noise 

•  Unstable Flow and Pressure & Trips may occur.

 

 

 

 

CHARACTERISTIC CURVE OF CENTRIFUGAL COMPRESSOR

 

Characteristic curve or Performance curve

Character or performance of equipment represented in the form of curves or graph.

Characteristic curve of centrifugal compressor

Centrifugal Compressor Performance or characteristic curve is the curve between Polytrophic Head and Inlet Volume Flow.

Performance curve also known as compressor map.

Sample compressor map for centrifugal compressor is as shown below.

 

 Boundaries of a centrifugal compressor map are shown in below figure.

                       

Minimum speed and maximum speed are the limits maintained by the driver of the compressor.

Maximum flow limit of the curve is known as stone wall or choke. It occurs at high flow and low pressure.

Minimum flow limit of the curve is known as stone surge. It occurs at low flow and high pressure.

Both stonewall and surge effects are the unstable operating points of a compressor.

Compressor shall be operated between all the limits and that zone is called stable zone of operation.

 

 

CENTRIFUGAL COMPRESSOR

Centrifugal

Move away from the centre.

Centrifugal compressor

Compress fluid by using the principle of dynamic compression.

Fluid enters at the centre of the impeller i.e. impeller eye along its axis and exits along the circumference through the impeller vane tips.

Inlet guide vanes are installed before fluid entering the impeller which can be used for compressor control.

The part of the impeller which penetrates the inlet nozzle end is called the inducer. The inducer blades are angled towards the direction of rotation. In the centrifugal section of the impeller, fluid enters from the inducer and exit the impeller radially as mentioned above. The blades at the exit may be curved in a backward- or forward swept manner, away from or against the angle of rotation.         

Then the fluid flows into the diffuser, where the velocity is reduced in order to gain pressure. The diffuser channels are therefore usually diverging. Pressure increase also occurs due to redirection of flow.

Types of compressors

·         Single stage compressor

·         Multi stage compressor

Main components of centrifugal compressors

·         Impeller

·         Diffuser

·         Shaft

·         Gearbox (Gears and pinions)

·         Bearings

·         Seal housing.

 

 

 

COMPRESSORS

 

Introduction

Compressor – A Mechanical device used for following function.

Function

 Increase the pressure of the fluid (gases) and transport from one location to other.

Applications

Gas Transport, Power generation, Refrigeration & Air conditioning, Refineries / Oil & Natural gas processing plants / petrochemical & chemical plants for product  processing applications, Air craft engines, etc.

Principle

1.       Increase in pressure by reducing the volume of the gases. (Positive displacement compression)

2.       Increase in fluid velocity, followed by a reduction of velocity to increase the fluid pressure. (Dynamic compression)

Main types of compressors & selection criteria

1.       Reciprocating compressors:         High pressure Low flow and service.

2.       Centrifugal compressors:             Medium pressure and medium flow service.

3.       Axial compressors:                       Medium pressure and high flow service.

 A compressor selection criterion is not only based upon pressure and flow, but also suction temperature, gas composition & mode of operation shall be taken into consideration.

  

                Classification of compressors