PUMP TRAINING





FREE PUMP TRAINING (Part2)
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READING CENTRIFUGAL PUMP CURVES
(Refer to Fig.2 below and its notes
)

Centrifugal pump performance is represented by multiple curves indicating either:

  • Various impeller diameters at a constant speed.

  • Various speeds with a common impeller diameter.

The curve consists of a line starting at "shut head" (zero flow on bottom scale / maximum head on left scale). The line continues to the right, with head reducing and flow increasing until the "end of curve" is reached, (this is often outside the recommended operating range of the pump).

Flow and head are linked, one can not be changed without varying the other. The relationship between them is locked until wear or blockages change the pump characteristics.

The pump can not develop pressure unless the system creates back pressure (ie: Static (vertical height), and /or friction loss). Therefore the performance of a pump can not be estimated without knowing full details of the system in which it will be operating.

Following is fig.2 and its notes (NOTE: text colours relate to colours used in fig.2):
Best efficiency point

Various performance curves, may indicate various impeller diameters, or speeds.

Curves showing power absorbed by pump (may be a separate line), read power at operating point, see note 1.

Recommended operating range.

Nett positive suction head required by the pump (lines may be shown intersecting pump curve)- should be 1m less than NPSH(a)


The points referred to as "shut head: and "end of curve" which are outside pump operating range.

The circled numbers indicate the following for bottom curve (ie: smallest diameter impeller or slowest speed curve shown):

1. Maximum recommended head.

2. Minimum recommended head.

 


3. Minimum recommended flow.

4. Maximum recommended flow.


Note 1: SELECTING MOTOR SIZE:
Power absorbed by pump is read at point where power curve crosses pump curve at operating point, (or curve separate to pump curve). However this does not indicate motor / engine size required. Various methods are used to determine driver size.
  1. Select motor or engine to suit specific engine speed or operating range - most cost effective method where operating conditions will not vary greatly. (very risky - as a pump is often not used for the purpose it was originally intended)

  2. Read power at end of curve - most common way that ensures adequate power at most operating conditions.

  3. Read power at operating point plus 10% - usually only used in refinery or other applications where there is no variation in system characteristics.

  4. By using system curves all operating conditions can be considered - best method where filling of long pipelines, large variations in static head, or siphon effect exist. -use orifice plates or valves to control power usage.


CENTRIFUGAL PUMP OPERATING RANGE

All types of pumps have operational limitations. This is a consideration with any pump whether it is positive displacement or centrifugal. The single volute centrifugal pump (the most common pump used worldwide) has additional limitations in operating range which, if not considered, can drastically reduce the service life of pump components.

"BEP" - Best Efficiency Point (refer to fig.3 below) is not only the operating point of highest efficiency but also the point where velocity and therefore pressure is equal around the impeller and volute. As the operating point moves away from the Best Efficiency Point, the velocity changes, which changes the pressure acting on one side of the impeller. This uneven pressure on the impeller results in radial thrust which deflects the shaft causing:

  • Excess load on bearings.

  • Excess deflection of mechanical seal. or;

  • Uneven wear of gland packing or shaft / sleeve.

The resulting damage can include shortened bearing / seal life or a damaged shaft . The radial load is greatest at shut head.

Outside the recommended operating range damage to pump is also sustained due to excess velocity and turbulence. The resulting vortexes can create cavitation damage capable of destroying the pump casing, back plate, and impeller in a short period of operation. Refer to fig.3 which indicates range of operation (between approximately 50% and 120% of Best Efficiency Point)

When selecting or specifying a pump, it is important not to add safety margins or base selection on inaccurate information. The actual system curve may cross the pump curve outside the recommended operating range. In extreme cases the operating point may not allow sufficient cooling of pump, with serious ramifications!

The best practice is to confirm the actual operating point of the pump during operation (using flow measurement and / or a pressure gauge) to allow adjustment (throttling of discharge or fitting of bypass line) to ensure correct operation and long service life.





SYSTEM CURVES

System curves allow correct selection of pumps and pipes, and are invaluable in troubleshooting pump and system problems. To draw a system curve, follow these steps & refer to fig.4:

  1. Find details of duty. ie, in this example: Water, 2m suction lift, 15m static discharge (17m total static head), 360 metres of 150mm schedule 40 steel pipe.
  2. Draw a chart with flow on bottom scale and head on left scale. (estimate scale required based on size of existing pump, or guess maximum flow expected - example shows max flow as 100 L/S and max head as75m - sometimes you just have to guess to get started)
  3. Mark static head. ie: 17m at zero flow.
  4. Mark 2 or 3 other points.ie: at 20L/S friction loss is 0.73 m / 100m of pipe, therefore 0.73 x 3.6 + 17 = 19.6 metres. Put mark at junction of 20 L/S and 19.6 m. Repeat for other points. (remember to add static head each time)
  5. Join these points with a line. You have completed the System Curve. (Curve may have to be extended to suit higher flow pumps.)
  6. The pump operating point is where a pump curve crosses the system curve. Draw as many pump curves over the system curve as you like, to see where different pumps will operate, or draw system curve over pump curve.
  7. If pump curve does not cross system curve, the pump is not suitable. If the pump curve crosses the system curve twice, then the pump will be unstable and is not suitable.
Note: It is tempting to add extra margins to these calculations, but in most circumstances that can contribute to the wrong pump selection and a big repair bill for a damaged oversized pump operating outside it's operating range. Extra Note: Some applications require a best guess (hopefully) oversize pump with budgeting for pressure gauges, valves, and / or orifice plates, to allow adjustment during commissioning to ensure the duty is achieved with maximum pump life.
Note: 'demand' pressure, ie: sprinklers etc, should be added at each flow calculated to make the system curve. If you can't get the data for the sprinkler / nozzles at various flows, but you know that ie: 10 sprinklers will pass 0.2 L/s each at approx 30psi, then your required flowrate is 2 L/s and add 21m (approx 30 psi) to the static head when you start drawing the system curve (as an approximation).

The 'demand' pressure or "head loss through sprinkler / nozzle at a particular flowrate" is not added for each sprinkler. Only the flowrate of each one is added together. If the pressure is available for the most 'disadvantaged' sprinkler, then it will be available for all sprinklers in that system. Note: this means that a higher pressure will be available to less 'disadvantaged' sprinklers allowing a higher flowrate through those sprinklers. There may be no need to calculate each sprinkler / nozzle, but if there are significant differences in static head / long lengths of pipe / reduced pipe diameters, then the system may require more investigation to allow correct pump selection.


PARALLEL & SERIES OPERATION

The use of two or more pumps to increase flowrate is called Parallel pumping. The use of two or more pumps to increase head is called Series pumping. Operation of pumps under these circumstances may appear simple, but there are more complex issues to consider, ie:


In series applications: consider the pressure rating of pump, shaft seal, pipework and fittings. Placement is critical to ensure both pumps are operating within their recommended range and will have a constant supply of water.

Drawing a curve for 2 or more pumps is simple, draw 1st pump curve then draw 2nd curve, adding the head each pump produces at the same flow. More curves can be added in the same way Fig.5.

 
    


In parallel applications: confirm suitability of pumps by drawing a system curve (often 2 pumps will only deliver slightly more than one pump due to excessive friction loss. Also you can confirm that pump operation will be within its recommended range.). Non return valves are required especially if one pump operates alone at times. Dissimilar pumps or pumps placed at different heights requires special investigation.

Drawing a curve for 2 or more pumps is simple, draw 1st pump curve then draw 2nd curve, adding the flows each pump delivers at the same head. More curves can be added in the same way Fig.6.

Once the curve for two pumps has been drawn, add the system curve, the point where the system curve crosses the curve for two pumps , indicates the total flow from two pumps. Draw a horizontal line from this point back to the head axis. Where this horizontal line crosses the curves for a single pump indicates the amount of flow contributed by that pump to the total flow.

Unstable Operation

Fig.7 shows a system curve crossing a pump curve twice. This is an example of unstable operation. Note that if the first pump is operating at point 'C' when the second pump is started, the second pump will operate at shut head, delivering no flow as it will never be able to open the non return valve (required to prevent one pump discharging through the other when only one pump is operating). If this was to occur, the pump could eventually explode!

In some cases it may be possible to change the order of starting the pumps, and the curves can be drawn to check this operation, however if there is any indication of unstable operation or possibility of one pump being 'over powered' by another, the system may need to be changed or different pumps will have to be used.



CAVITATION - TWO MAIN CAUSES

A: NPSH (r) EXCEEDS NPSH (a)

Due to low pressure the water vapourises (boils) and higher pressure implodes into the vapour bubbles as they pass through the pump causing reduced performance and potentially major damage. For more see NPSH - Nett Positive Suction Head

 

B: Suction or discharge recirculation

The pump is designed for a certain flow range, if there is not enough or too much flow going through the pump, the resulting turbulence and vortexes can reduce performance and damage the pump. For more see Centrifugal pump Operating Range


The damage to a pump from cavitation can be severe. It may shorten seal and bearing life and damage volute, backcover, and even pipework beyond repair. The implosions due to cavitation can sound like gravel passing through the pump. Pumps are not able to pass vapour or air same as an air compressor is not able to pass water.


NPSH - Nett Positive Suction Head

NPSH is a dirty word? There is enough fear of it to suggest it is. But why? Because some people will not accept that pumps don't suck!
If you accept that a pump creates a partial vacuum and atmospheric pressure forces water into the suction of the pump, then you will find NPSH a simple concept.

NPSH(a) is the Nett Positive Suction Head Available, which is calculated as follows:

NPSH(a)= p + s - v - f

Where:
'p'= atmospheric pressure,
's'= static suction (If liquid is below pump, it is shown as a negative value)
'v'= liquid vapour pressure
'f'= friction loss

     


NPSH(a) must exceed NPSH(r) to allow pump operation without cavitation. (It is advisable to allow approximately 1 metre difference for most installations)

NPSH(r) is the Nett Positive Suction Head Required by the pump, which is read from the pump performance curve. (Think of NPSH(r) as friction loss caused by the entry to the pump suction.)

The other important fact to remember is that water will boil at much less than 100 deg C if the pressure acting on it is less than its vapour pressure, ie: water at 95 deg C is just hot water at sea level, but at 1500m above sea level it is boiling water and vapour. There was enough atmospheric pressure at sea level to contain the vapour, but once the atmospheric pressure dropped at the higher elevation, the vapour was able to escape. This is why vapour pressure is always considered in NPSH calculations when temperatures exceed 30 to 40 deg C.



AFFINITY LAWS - CENTRIFUGAL PUMPS

If the speed or impeller diameter of a pump change, we can calculate the resulting performance change (at the same efficiency) using:

Affinity laws:
1. The flow changes in proportion to speed ie: double the speed = double the flow
2. The pressure changes by the square of the difference ie: double the speed = multiply the pressure by 4
3. The power changes by the cube of the difference ie: double the speed = multiply the power by 8




TROUBLESHOOTING

Only one thing is a better troubleshooting tool than flowmeter and pressure / vacuum gauges...that is:
readings from flowmeters, pressure / vacuum gauges taken prior to the problem. ie: monitoring.

Gauge readings will help diagnose pump and system problems quickly, by reducing the possible causes. Flow measurement would allow full diagnosis of pump performance but is sometimes expensive or not possible (Cheap versions include: V notch weir, measuring discharge from horizontal pipe, & timing of filling / emptying). System curves can be used in evaluating results.

Following is troubleshooting - this list is not extensive or complete, but covers basic symptoms and some possible causes:

1. Pump does not prime.

     

Suction lift too great.
Insufficient water at suction inlet.
Suction inlet or strainer blocked.
Suction line not air tight.
Suction hose collapsed.
Mechanical seal / packing drawing air into pump.
Dry-prime pumps - discharge non return valve leaking

2. Not enough liquid.

Incorrect engine speed.
Discharge head too high.
Suction lift too great.
Suction inlet or strainer blocked.
Suction line not air tight.
Suction hose collapsed.
Mechanical seal drawing air into pump.
Obstruction in pump casing/impeller.
Impeller excessively worn.
Delivery hose punctured or blocked.

3. Pump ceases to deliver liquid after some time.

Excessive air leak in suction line.
Mechanical seal / packing drawing air into pump.
Obstruction in pump casing/impeller.
Delivery hose punctured or blocked.
Suction lift too great.
Insufficient water at suction inlet.
Suction inlet or strainer blocked.
Suction hose collapsed.

4. Pump takes excessive power.

Engine speed too high.
Obstruction or contact between impeller and casing.
Viscosity and / or SG of liquid being pumped too high.
Bearing failure or severe coupling misalignment.

5. Pump vibrating or overheating.

Engine speed too high.
Obstruction in pump casing/impeller.
Impeller damaged.
Cavitation due to excessive suction lift / friction loss.

6. Pump leaking at seal housing.

Mechanical seal damaged or worn. Due to:
Dry Running during priming or loss of liquid.
Cracking of faces can occur due to thermal shock, after pump has run dry or against shut head, and then cool water enters the pump casing.



 

Use of this Pump Training website is subject to acceptance of the Disclaimer



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