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.

 

 

 

HYDRAULIC BALANCING OF CENTRIFUGAL PUMP – BALANCING DISC AND COMBINATION OF BALANCING DRUM & BALANCING DISC

                Balancing disc is a balancing device used for hydraulic balancing of multi stage centrifugal pump which is fixed to the shaft and rotates with it.

It is separated by a small axial clearance from balancing disc head which is fixed to the pump casing as shown in the following figure.

 

 The leakage of the fluid through this axial clearance flows into the balancing chamber and then to the suction or suction vessel through a pipe which is also called balancing line or balance leak off line.

As the fluid shall flow to the suction from balancing chamber to suction, the pressure in the balancing chamber slightly maintained higher than the suction pressure. This balancing chamber pressure in balancing chamber can be maintained using restricted orifice. Also balancing chamber pressure is one of the important parameters of the pump to monitor. If balancing chamber pressure is not maintained within its limits then there will be a provision to damage of balancing disc and same thing applies even to balancing drum.

The forces acting on the balancing disc are as follows.

1.       Back side of the balancing disc = Balancing chamber back pressure.

2.       Front side of the balancing disc or disc face = Experiences some range of pressures.

The inner and outer disc diameters are designed in such a way that the difference between total force acting on disc face and that acting on disc back will balance impeller axial thrust.

If the axial thrust of the impellers exceed the thrust acting on the disc during operation then the disc is moved towards the disc head and thereby reducing the axial clearance between the disc and disc head.

From reducing clearance, the mechanism of the balancing disc can be shown in following steps.

Reducing the axial clearance between the disc and disc head 

Leakage to the balancing chamber is reduced

                    
Pressure difference between balancing disc and balancing chamber increases

Disc moves away from disc head

        

Clearance increases

      

 Fluid leakage to balancing chamber increases

      

 Back pressure in balancing chamber increases

Disc again moves towards disc head until equilibrium is achieved

 

    The main drawback with balancing disc is the pressure on seal chamber is variable which is injurious to seal. The life of seal decreases.

To avoid the above drawback of balancing disc and to retain the advantage of automatic compensation for axial thrust changes, the combination of balancing drum and balancing disc is used.

For this combination, the rotating portion of this balancing device consists of a long cylindrical body that turns with in a drum portion of the disc head. This rotating part incorporates a disc similar to one previously described.

In this design,

Radial clearance remains constant irrespective of disc position and

Axial clearance varies with the pump rotor position.

The combination of balancing drum & balancing disc can be shown in below figure.

 

 The forces acting on this balancing device is as follows.

1.       Towards discharge end = {(Discharge pressure * Area A) + (Average intermediate pressure * Area B)}

2.       Towards back side of the balancing device = Back pressure * Area C

Based on the applications and the requirement, balancing drum or balancing disc or combination of both can be selected.

While assembling the balancing device, all the bolts shall be tightened with recommended torque or else the scoring will be resulted on balancing drum or balancing disc with balancing drum head or balancing disc head.

 

 

 

 

 

HYDRAULIC BALANCING OF CENTRIFUGAL PUMP – BALANCING DRUM

                In this post, the axial thrust forces distribution of the multi stage centrifugal pump and the concept of balancing drum will be discussed in detail. Below figure shows pressure distribution in a multi stage centrifugal pump.

 

To understand the concept of hydraulic balancing in multistage centrifugal pump, first we will consider the resultant pressure acting on the pump impellers can be understood by following cases.

Case I:  without installing any balancing devices.

From the above figure, it is clear that

The net pressure acting towards suction side = P5-P1.

The net pressure acting towards discharge side = P5. (Please not this is net pressure without installing any balancing devices)

To hydraulically balance on both suction side and discharge side, the net pressures acting on suction side and discharge side shall be made equal which can be done by installing balancing device.

Case II:  After installing balancing device

For this case also, the reference can be taken from above figure.

The net pressure acting towards suction side = P5-P1.

The net pressure acting towards discharge side = P5-Pb. (Pb is the balancing chamber pressure or pressure developed due to the design of balancing chamber and installation of the balancing device).

For perfect hydraulic balancing, P1 = Pb.

But in practical, Pb (balancing chamber pressure) shall be maintained in such a way that it is slightly greater than suction pressure as the fluid from balancing chamber will be going back to the suction side of the pump.

The balancing device shall balance this axial thrust and this hydraulic device may be balancing drum, balancing disc or combination of both.

The balancing drum is installed at last stage of the pump and is separated by a small radial clearance from the stationary portion of the balancing device called balancing drum head which is fixed to the pump casing.

Through the radial clearance, there will be fluid leakage and balancing chamber pressure is maintained based on the design parameters (dimensions) of the balancing drum.

The forces acting on the balancing drum are as follows.

1.       Towards the discharge end = Discharge pressure * Front balancing area (B) of the drum.

2.       Towards the suction end = Back pressure in balancing chamber * Back balancing area (C) of the drum.

Balancing drum diameter can be selected to balance the axial thrust completely or 90 to 95% depending on the desirability of carrying any thrust bearing loads.

The main drawback of the balancing drum is it lacks automatic compensation of any changes in axial thrust caused by varying impeller reaction characteristics.

For example, if axial thrust and balancing drum forces becomes unequal, then the rotating part will tend to move in direction of greater force. In this case, the thrust bearing must prevent excessive movement of rotating part.

Balancing drum performs no restoring function until such time as drum force again equals to axial thrust.

This drawback can be avoided using the design of the balancing disc.

 

 

INTRODUCTION TO HYDRAULIC BALANCING OF CENTRIFUGAL PUMP – MULTI STAGE

                Hydraulic axial thrust developed in multi stage pump will be sum of the axial thrust acting on the individual impeller which is very high in magnitude. To balance hydraulically for this high magnitude axial thrust, the special hydraulic balancing device used is balancing drum or balancing disc.  In some applications the combination of two balancing devices is used (balancing drum and balancing disc).

Balancing drum, balancing disc and a combination of both is shown in following figures.

 

                                                                        Fig: Balancing drum

                                                                Fig: Balancing disc

                                  Fig: Combination of both balancing drum and balancing disc

Balancing drum

Balancing drum is separated by a small radial clearance from the stationary portion of the balancing device called balancing drum head which is fixed to pump casing.

Balancing disc

Balancing disc is separated by small axial clearance from balancing disc head which is fixed to casing. It is fixed to shaft and rotates with it.

 Further details regarding balancing drum, balancing disc and axial thrust balancing forces in balancing drum and balancing disc in detail will be discussed later.

HYDRAULIC BALANCING OF CENTRIFUGAL PUMP – SINGLE STAGE

During operation of centrifugal pump, the hydraulic imbalance is caused due to the different values of pressures in the pump like discharge pressure and suction pressure acting on either side of the impeller and around circumferential direction of the impeller.

The following are the two hydraulic forces acting on the impeller.

Ø  Radial Thrust

Ø  Axial Thrust

Bearing life of the centrifugal pump also depends on the above two hydraulic forces.

Radial Thrust

The force generated in radial direction due to unequal pressure generation in volute is called radial thrust.

Radial thrust can be reduced by

ü  Double volute casings

ü  Diffuser type casings

 

 Axial Thrust

The force generated in axial direction due to unequal pressure generation on either side of the impeller is called axial thrust.

Axial thrust can be carried away by thrust bearings.

  Axial thrust can be reduced by

ü  Provision of balancing holes on pump impeller

ü  Provision of back vanes or back plate on the impeller

Provision of balancing holes on pump impeller

·         Drilling hole through impeller back shroud into front portion of impeller. There by pressure distribution is in such a way that axial thrust is balanced up to some extent.

Figure below shows the pressure distribution of the fluid and reduction in axial thrust in the centrifugal pump when balancing hole is provided. 

·         Leakage through holes is directed against the flow in the impeller eye causing disturbances.

·         But balancing by this method will not fully reduce axial thrust.

Provision of back vanes or back plate on the impeller

·         Radial vanes are used on back shroud of the impeller to reduce the pressure in the space between the impeller and pump casing.

·         Radial back vanes provided at back shroud of impellers acts as an auxiliary impeller which restricts the entry of the liquid into the clearance between impeller back shroud and casing cover.

·         Balancing by this method reduces axial thrust in a better way when compared to provision of balancing holes.

Figure below shows the pressure distribution of the fluid and reduction in axial thrust in the centrifugal pump when back vanes are provided. 

Pic credits

Axial Thrust in centrifugal pumps - Experimental analysis (15th international conference on experimental mechanics)

 

CENTRIFUGAL PUMP COMPONENTS

The following are the major components of a centrifugal pump and are shown in following figure.

                    

                     

Pump volute casing

Casing that receives the fluid being pumped by the impeller. The shape of the volute casing is in such a way that the cross sectional area gradually increases towards discharge resulting in the conversion of kinetic energy to the pressure energy at the discharge.

Impeller

Impeller is the main part of a centrifugal pump that pumps the process fluid from suction side to discharge side. It consists of equally spaced vanes around the impeller eye. Impeller eye is the entry point at which fluid enters into the impeller.

Shaft

The main function of the shaft is to transmit the torque and supports the impeller and all other parts of a centrifugal pump.

Key

Key is a metallic part which serves as a locking mechanism between the two rotating parts like impeller and shaft etc. It is installed in a groove called key way.

Shaft sleeve

Shaft sleeve is used to protect the shaft from mechanical damage caused by the seals or bearings and chemical damage caused by the aggressive fluids.

Bearings

Bearings give support to the shaft in rotation and also reduce frictional forces. In centrifugal pump bearings are usually designated as inboard bearing and outboard bearing.

Normally both radial bearings and thrust bearings are used in a pump to compensate for both radial and thrust loads.

Mechanical seal or gland packing

Mechanical seal or gland packing prevents leakage of the process fluid into atmosphere at a point where the pump shaft passes out through the casing.

Considerable minimum leakage is allowed in gland packing.

Approximately zero leakage is allowed in mechanical seal.

Gland packing is used in non hazardous services and mechanical seal is used for hazardous fluids where no fluid is allowed to leak into atmosphere.

Lantern ring

The lubricating or cooling fluid supplied to packing is distributed through lantern ring. It is commonly a brass ring with two holes that allows the lubricating or cooling fluid to easily pass through.

Bearing isolator

The bearing isolator solves the purpose of the following.

ü  To avoid the passage of process fluid to bearing housing.

ü  To avoid oil leakage from oil sump to outside of bearing housing.

ü  To avoid dirt into housing in dirty areas.

Deflector

If mechanical seal leaks then the deflector deflects the process fluid perpendicular to the shaft to avoid mixing with bearings and oil. It is generally made up of rubber or plastic.

Oil ring

Oil ring or oil flinger is used to circulate the oil in the oil sump.

Wear rings

These are placed at impeller and casing as to avoid the damage of the impeller and casing. Impeller wear ring gets damaged before the impeller damage and casing wear ring gets damaged before casing damage. These are easily replaceable parts.

Breather

It is a special part which let out the vapours liberated in the oil sump. Allows the air to enter and exit the lube oil sump without particulate contamination.

Gaskets

Gaskets prevent the leakage of the process fluid from wet parts of the pump to atmosphere.

These are placed mainly at five positions.

1.       Between impeller and impeller nut.

2.       Between impeller and shaft sleeve.

3.       Between back plate and casing.

4.       At drain plug.

5.       Between mechanical seal gland and seal.

The following are the wet parts of the pump.

ü  Volute casing

ü  Impeller nut

ü  Impeller

ü  Wear rings (casing wear ring and impeller wear ring)

ü  Back plate

ü  Shaft sleeve

ü  Mechanical seal or gland packing

ü  Seal gland

Throat bush

It serves as the restriction for the fluid flow between impeller and the mechanical seal area.

Constant level oiler

It acts as a lube oil reservoir to the lube oil sump and used to maintain a required level of oil in the lube oil sump for lubrication.

Bearing Housing

Bearing housing support bearings and protect from contaminants for lubrication.