SECTION 1 : INTRODUCTIONPumps

Pumps provide a means of adding energy to the fluid in order to have the capability of transporting fluid from one level of potential and kinetic energy to another. Depending upon the multitude of parameters various means of adding energy to the fluid are employed.

Some of the most prominent considerations in selection of pumps are Capacity Differential pressure Temperature Fluid characteristics Pressure Specific to system

The centrifugal pumps are most versatile and are hence extensively used in industries. Out impetus would hence be on centrifugal pump alone.

Basic Parts of a Centrifugal Pump

Impellers: It is the rotating part of the pump through which liquid passes and which imparts the energy to the fluid. Impellers are of various types:

1. Fully enclosed - used for high head, high pressure applications.2. semi enclosed – General purpose applications, has open vane tips at entrance to break

up suspended particles and prevent clogging.3. open – used for low head, suspended solids applications, very small flows.

Casing:It is the housing surrounding the impeller. It contains bearings for supporting the shaft on which the impeller is mounted.

Packing:Material used to control leakage between stationary and moving parts. Alternatively, mechanical seals are mounted on the shaft of centrifugal pumps to minimize casing leakage. They are preferred because of the longer life and minimized leakage.

Pump hydraulics Vs Cardio vascular systemWater hammer

1. Start, stop or an abrupt change in a pump’s speed.2. Power failure.3. Rapid closing of a control valve.

Hydraulic characteristics for centrifugal pumps

Capacity:The rate at which the liquid flows through the pump commonly expressed in actual volumetric flow per hour (m3/hr).

Total Head:The pressure available at the discharge of a pump as a result of the change of mechanical input energy into kinetic and potential energy. This represents the total energy given to the liquid by the pump.

The total head read on the pump curve is the difference between the discharge head (the sum of the gauge reading on the discharge connection on the pump outlet, for most pumps corrected to the pump centerline, plus the velocity head at the point where the gauge is attached) and the suction head (the sum of the suction gauge reading corrected to pump centerline and the velocity head at the point of attachment of the suction gauge).

This head produced is independent of the fluid being pumped. Therefore for a given pump impeller diameter and speed, the pump will raise the liquid to a certain height regardless of the weight of the liquid (refer fig 1.1).

NPSH:Net positive suction head above the vapour pressure of the liquid at the pumping temperature is the absolute power at the pump suction. It is very important while handling liquids closer to their boiling point or liquids of high vapour pressure.

Cavitation: Pressure drop occurs between the suction flange and the pressure point within the impeller. Pumping system must have a positive pressure to overcome this pressure drop. If pressure is lesser than the vapour pressure liquid would vapourize and vapour bubbles get carried to a point of higher pressure and collapse. This phenomenon is referred to cavitation.

Fig . 1.1

The NPSH available for the system is given by the equationNPSH avl = S +(Pa –Pvp) – hslS- static headPa – Absolute pressure in suction vesselPvp – vapour pressure of the liquid at operating temperaturehsl – Suction line losses

It is essential that the available head calculated above should be more than the required NPSH atleast by 1m.

The required NPSH is function of physical dimensions of the casing, specific speed and type of impeller. The value is given by the pump manufacturer.

Pump efficiency:It is the ratio of the Brake horse power to hydraulic power. The brake horse power is lesser than hydraulic power due to hydraulic and mechanical losses in the pump.The efficiency increases with capacity up to a certain capacity (best efficiency) and then decreases with further increase in capacity.

WP = Pump work per unit mass of fluid, J/kg.WP-hfp= WP

= efficiency of the pump

hfp = friction loss in pump.

Fig 1.2

Fig. 1.2 shows a simple pumping loop.

Pump Characteristic curve :Performance of a pump is characteristic of its dimensions. The pump manufacturer gives the performance characteristic of the pump. A typical curve shows the capacity Vs head, power, NPSH required and pump efficiency. (Refer fig.1.3)

FANS LAWS ( AFFINITY LAWS)

The affinity laws relate the performance of a known pump along its characteristic curve to a new performance curve when either speed or impeller diameter is changed. It would represent a family of curves.

Fig. 1.3

1. For change in speed with similar impeller diameter pumps,

Q2/Q1 = (N2/N1) , H2/H1 = (N2/N1)2 , BHP2/BHP1 = (N2/N1)3

2. Similarly if impeller diameter is varied keeping speed constant,

Q2/Q1 = (D2/D1),H2/H1 = (D2/D1)2 , BHP2/BHP1 = (D2/D1)3

Where,

Q : volumetric flow rate through the pumpN: Speed of rotationH: Pump headBHP : brake horse powerSuffix 1& 2 refer to the existing and new pump respectively.

SECTION 2 : PUMPING SYSTEM ASSIGNMENT

In this section, the objective of the pumping system assignment has been discussed.

In any industry and especially in a refinery like ours, transportation of fluid from one area to another is a necessity. These as already explained are done by pumps. All the units in our plant strive to increase their capacity so as to give additional profit to the company. To achieve these capacities , all the equipment need to be adequate. Pumps form an integral part of each unit. It is hence important to check for the adequacy of the system at higher plant load or for any deviations in operating parameters from the current scenario.

The adequacy of the pumping system is checked extensively with help of isometrics of the system. The pumping system includes (refer fig. 1.2) ,1. The pump2. Motor or driver3. Control valve in the loop

The adequacy of the system is checked for1. Pump’s deliverable head, NPSH and capacity2. power deliverable by the motor3. available head across control valve (adequacy of control valve)

In cases where any of the above is found to be inadequate for the new conditions, suitable options to overcome the inadequacies the same are suggested.

SECTION 3 : DATA COLLECTION

Data collection is the first step in the progress of the assignment. It is essential to have a good knowledge of the pumping system in study. A pumping system may have more than one discharge points. It is necessary to check the adequacy of pumping system with regards to all loops.

Before checking for the adequacy of the pumping system we need to check the reliability of the working sheet that has been prepared. Hence validation of the calculation is done with current operating data.

The various component of the pumping loop are-Suction vessel-Suction strainer-Pump-Motor-Control valve-Exchanger/condenser-Any other equipment in the loop like drier, treater -Discharge vessel

Data collection can be broadly classified into-1. Collection of equipment data sheet2. collection of operating data

Collection of equipment data sheet:Vendor data sheet for pump, motor, control valves, exchangers and any other equipment in the loop is collected from Rdocs.The other data that need to be collected from Rdocs include-1. Pump performance curve (test curve/predicted performance curve)2. Control valve characteristics curve (using Fischer manual)3. Flow element data sheet (to know design flow and pressure drop)4. Elevation drawings for suction/discharge vessels (mechanical drawing )5. Level gauge data sheet (to know the distance between the two tapping)

Collection of plant operating data:The plant data that need to collected include-

Data to be collected Data source Remarks1. Suction vesselPressure IP 21, Field Pressure at the point from where the suction

line is drawn is to be checked (top/ bottom/middle for columns)

Level IP 21, Field Level in % needs to be changed to meter.2.PumpFlow through the pump IP 21, Field The observed flow needs to be corrected for

pump operating conditionsSuction temperature IP 21/FieldDischarge pressure Field Measurement is to be done with calibrated

gaugeFlowing fluid density LIMS This needs to be corrected for operating

temperature3. MotorAmperage, voltage Field reading &

substation valueReading to be noted when fluctuation in gauge is least. Compare this with that read from substation.

4. control valveValve opening Field & IP 21 Field opening needs to be cross checked with

IP 21 valueFlow through the valve Field / IP 21/ by

calculationIn case of more than one loop flow through each valve is to be determined

5. Discharge vesselPressure IP 21, Field Pressure at the point the discharge line

empties(top/ bottom/middle for columns)Level IP 21, Field Level in % needs to be changed to meter.

SECTION 4 : VALIDATION & ADEQUACY CHECK

Validation of any pumping system includes,1. Pump head2. Brake horse power3. Control valve opening

Using the above data in the working sheet , the actual pump head, BHP and control valve opening observed in the field are checked for reproducibility. Pump head:Two sources to have pump head are pump discharge pressure from filed and pump head from pump characteristic curve. The pump head from discharge pressure is calculated as follows:

Diff. Head = (Discharge Pressure) - (Suction vessel pr) - (liquid level in suction vessel) - (suction line losses)

This is compared with the head obtained from characteristic curve for the particular flow at corresponding impeller diameter (refer fig. 1.4 ).

BHP:Two sources for obtaining BHP are using motor reading noted in field and BHP read from pump characteristic curve (refer fig 1.4).

The BHP using field amperage and voltage is obtained by,BHP = 3* VI cos * Effm Where,V- voltageI- amperagecos- Power factorEffm - Motor Efficiency Power factor and motor efficiency are obtained from motor data sheet.

Control valve opening:The opening across the control valve is obtained by calculating the pressure drop available across the control valve.Del P (Avail) = (Pump head) – (line losses) – (equipment losses) – (static losses)

The Cv that corresponds to the available pressure drop and flow through control valve is calculated as,Q = 0.86 * Cv * (del P/)Where ,Q – Flow through the control valve - Density of the fluid flowing through control valve

With the valve characteristic curve, the valve opening corresponding to Cv is obtained.The above value is compared with the control valve opening observed in the plant.

Deviations within the following limits are considered acceptablePump head : 2%BHP : 2%Control valve open : 5% points.

Points to be considered while validating & checking adequacy :It is essential to note that with change in temperature along the loop, the density of the flowing fluid changes and hence the pressure drop changes. Hence, while considering any loop where process conditions change due to change in temperature (due to presence of exchangers/ condensers/heaters etc) proper density should be used for each section in the working sheet.

Phase changes due to presence of exchangers/condensers/heaters/ addition of liquid stream into a gaseous stream, need to be checked and accounted.

Pressure drops across exchangers/air coolers should be used from HTRI/HTFS if data is available. Square of flow relation using design data should be used only in cases where the above is not available. Values from HTRI/ HTFS give more realistic picture especially with exchangers which are prone to choking.

Fig 1.4

Q=0.86 CV (P/)1/2

Where,Q= m3/hrP=bar=Specific gravity

One CV is defined as one US gallon (3.78 litres) of 60°F water that flows through an opening, such as valve during 1 minute with a 1 Psi (0.1 bar) pressure drop.

Dynamic loss = Line loss + Loss across equipmentLine loss = 4fLv2/2gD

= 4fLQ2/2gDA2

Where,f= Friction Factorv=velocity of fluid.L= Length of pipelineD=Diameter of pipeline

This clearly shows that line pressure drop is proportional to square of flow.

Bernoulli’s EquationBetween point a and b,

WP = Pump work per unit mass of fluid, J/kg.

WP-hfp= WP

= efficiency of the pump

hfp = friction loss in pump.

hf = friction loss in pipe from a to b.

(Pa/) + (gZa/gc) + (Va2/2gc) + WP = (Pb/) + (gZb/gc) + (Vb

2/2gc) + hf

With point a as datum,

Pa = Pb

Va is negligible

Za = 0

WP = (gZb/gc) + (Vb2/2gc) + hf

Between A and B,

(Pa/) + (Va2/2gc) + WP = (Pb/) + (Vb

2/2gc) [hf =0, Za=Zb]

(Pb-Pa)/ = (Va2 – Vb

2)/2gc + WP

Pumps in series and parallelFlow Correction:

1) Calculate °API based on lab density.

2) Get Volume correction factor from Maxwell (STF library).

3) Get corrected specific gravity as lab density / volume correction factor.

4) Corrected Flow = (design density/corrected density) 1/2*observed flow.

Type of control valves :

1) EQ% characteristic

(dQ/dL) = nQ where, n is constant

If piping and downstream equipment provide significant resistance to the system.

High pressure drops with low flows and vice versa.

2) Linear characteristic

(dQ/dL) = k where, k is constant.

Most of the pressure drop is taken through the valve.

Upstream pressure is constant.

When variable head flowmeter is installed in the system.

METHODOLOGY (Pumping system)While checking the adequacy of the pumping systems in any unit, calculations for the control

valve opening, power consumption and head developed by the pump are validated for data

obtained during the normal operation of the plant. The adequacy is then checked w.r.t. NPSH,

head and power at higher load. For systems found to be inadequate at higher throughput,

suitable suggestions have been provided to make it adequate.

4.1 Validation of hydraulic calculation The operating data (flow rate, pressure, temperature, control valve opening) is collected from

field, DCS and IP.21. All loops in a given pumping system should be considered. The average values of the control valve openings for the given period is noted from IP.21.

This is independently cross checked with the opening observed in the field. Whenever a

bypass valve is kept open, it is mentioned in the respective annexures.

Static pressure drop is calculated which includes the static head and the pressure difference

between the discharge and suction vessel.

The length of pipe and fittings in the line is identified from the PIDs and isometric drawings.

They are converted into the total equivalent length.

The line loss in the pumping loop is calculated using the standard formulae. The total

dynamic loss includes the line losses and loss across equipment for both suction and

discharge side.

Wherever available, the pressure drops across the heat exchangers is taken from actuals/HTFS/HTRI (estimated for observed plant operation). In the absence of same,

they are estimated from design data i.e. calculated pressure drops (& not allowable pressure drops) on the appropriate side of the exchanger (shell / tube) are extrapolated on the basis of square of flow.

The discharge pressure of the pump has been noted in the field. Using this, the actual

differential head of the pump is calculated and compared with the differential head obtained

from the curve. Deviation upto +/- 2% of actual head from expected value of head is considered acceptable. Positive deviation here indicates pump underperformance w.r.t. head. The lesser of the two values has been considered for adequacy check at higher

throughput. Differential head obtained from the curve is used for those pumps where

pressure gauges are not installed in the running pump.

The head available across the control valve is obtained by subtracting the total dynamic

losses and static pressure drop from the head developed by the pump. This is done for all loops in a given pumping system.

If the predicted opening of the control valve is within +/- 5% range of the actual observed

opening in the field, the calculations are considered to be valid.

Motor related data is collected from substation. Using this data, the actual power consumed

by the pump is calculated.

The theoretical power consumption is calculated from the pump curve. This is compared with

the actual power consumption. Deviation up to +/- 5% of actual power from expected power is considered acceptable. Negative deviation here indicates underperformance w.r.t. power consumption.

4.2 Adequacy checks at target plant load: 4.2.1 Expected flows at target case:

Adequacy has been checked for all loops in a given pumping system.

If the flows thru a loop is governed by alternative scenarios (e.g. all flow diversion to tank in

case of emergency or change of flow split between 2 or more loops etc), the same is

considered. If flows vary at SOR, EOR or any other case, most demanding case has been

considered.

For adequacy check at target load, the flow rates are obtained on best possible data basis,

either thru simulation / kinetic model or by linear extrapolation of observed data.

The flowing specific gravity has been considered to be same as observed in current plant

operation.

4.2.2 Expected static pressures:

While estimating static pressure drop for each loop in a pumping system, following checks are

done.

Confirmation of source and destination vessels

Confirmation of expected pressures of source and destination vessels (pressures w.r.t. worst

scenario is considered and may not be necessarily same as that observed in current plant

operation).

Correct elevations of source and destination vessels from the pump centerline are used. Worst

operating scenarios (e.g. product being pumped to almost full tank, feed coming from tank,

which is almost empty etc.) are considered. While converting the height of liquid (m) to

pressures (Kg/cm2), correct density at the respective vessel is used.

Considering worst scenarios, the level of suction/source vessel is taken at LAL of the LT span

while that at destination vessels, it is taken as HAL.

4.2.3 Parameters for adequacy check:The pumping system adequacy is subdivided into:

A. Adequacy of NPSH

B. Adequacy of head

C. Adequacy of motor

D. Line size adequacy

A. Adequacy of NPSH availableNPSH available is calculated taking into account all the relevant factors i.e., suction vessel

pressure, suction side frictional losses, minimum height of the liquid above the suction of pump,

and vapour pressure of the fluid at pump suction. Though the static pressure drop considers LAL

value for suction vessel, for conservative purpose, while estimating NPSHA , the suction vessel

level is considered zero. Elevation credit is only taken from bottom tangent level of the suction

vessel. It is compared with the NPSH required from the pump curves. If the NPSHA >NPSHR at

least by 1 metre, then the system is considered to be adequate w.r.t. NPSH. For NPSHA check at

higher load condition, minimum level in the suction vessel has been taken as 20%.

In case of NPSH inadequacy, options like increasing suction vessel liquid level, increase of

suction vessel pressure, decrease in temperature at pump suction has been considered.

B. Adequacy of Head The actual head developed by the pump (as per field measurement) has been compared with

the head obtained from pump curve. The least of these two values (worst case) is considered

for adequacy check. If the actual head is lesser than that obtained from pump curve, (i.e.

positive deviation from expected head), the % deviation has been carried forward while

checking the adequacy of the pumping system at higher throughput. As a conservative basis,

this is also been applied even if impeller change is suggested.

The head available across the control valve is obtained by subtracting the total dynamic

losses and static pressure drop in the system from the differential head provided by the

pump.

Pressure drops for exchangers are taken from exchanger simulations. If it is not available,

actual pressure drops or design pressure drops (calculated values) are extrapolated by

square of flow. The same procedure is followed for other equipment like reactor, heater etc.

For filters, the maximum design pressure drop in dirty conditions (as quoted by vendor) is

used.

Pressure drop across flow measurement element (orifice etc) is also checked. If there is

abnormal increase, reboring of orifice is suggested to maintain pressure drop in normal

range.

The required head across the control valve is calculated for the corresponding flow through

the valve and maximum CV value. The maximum CV is considered as the CV that corresponds

to 80% of valve opening for linear valves and 90% opening for equal percentage valves.

The required head is compared with the available head across the control valve.

If the available head across the control valve is more than the required head, the system is

considered to be adequate w.r.t. Head.

Following de-bottlenecking options have been considered in case of head inadequacy. Refer

Annexure 1 for selection of these options.

1. If the available head across the control valve is less than that required and if the available

head is more than 15% of the dynamic losses, then the adequacy check is repeated with

either trim change of control valve or with higher size control valve.

2. If the available head across the control valve is less than 15% of the dynamic losses, then

adequacy check is repeated with new higher impeller diameter

3. If the head requirement is still not met, a check is done when both the impeller and control

valve are changed.

4. An addition of parallel pump having same impeller diameter/ higher impeller is suggested, if

the above cases fail.

5. In situations where none of the above options is satisfactory, replacement of the existing

pumps with new pumps is suggested.

6. Another option of changing pipeline diameter is also studied where dynamic losses are high

and cause bottlenecks. The change is suggested provided the change reduces the dynamic

losses considerably.

7. After suggested modifications, the head available across the control valve is re-checked to

confirm that it is at least 15% of the total dynamic losses so that controllability is ensured.

C. Adequacy of motor rating available

The BHP required is calculated for the desired flow and corresponding head from pump curves. It

is compared with the driver KW available given in the pump data sheet. If the margin available

between BHP and driver KW is less than that recommended as per API 610, then a suitable

higher rating motor is recommended. The motor is considered adequate if

The available driver rating KW is at least 25% higher than the BHP required, for motors with

rating <22 KW.

The available driver rating KW is at least 15% higher than the BHP required, for the motors

with the ratings 22-55 KW.

The available driver rating KW at least 10% higher than the BHP required, for the motors with

the ratings >55 KW.

If observed deviation in power consumption indicates underperformance (i.e. negative

deviation from expected power), it is checked that the available/suggested motor is adequate

after considering such deviation.

D. Line velocities and pressure drops: The line velocity and pressure drop in the pipe sections is checked and compared with the UOP

line sizing criteria.

Typical limits for velocity and pressure drop as specified by UOP are:

1. suction line velocity : 1.7 m/s

2. Discharge line velocity : 3.6 m/s

3. Suction line pressure drop :0.08 Kg/cm2 / 100 m

4. Discharge line pressure drop :0.36 Kg/cm2 / 100 m

The above limits are considered as a ‘guideline’ or ‘Soft limit’ only. Wherever the velocity and

pressure drop/100m are higher than that of UOP specified values, it has been highlighted in the

respective annexures.

4.2.4 Checks on downstream equipment for modified pumps:

For making a pumping system adequate at target load, if suggested modifications lead to

change in shutoff pressure of a pump, the effect of the same on downstream equipment

(before control valve) is checked. For this, the new shut off pressure is compared with design

pressure of the downstream equipment, to ensure that the former is less than latter.

In case the shut off pressure exceeds the design pressure of downstream equipment,

alternative modification is suggested.