Pertinent Characteristic of Pumps

Pertinent Positive Displacement Pump Characteristics

1. The discharge pressure produced is a function of the system requirement only and is independent of pump capacity.
2. The flow rate pulses between maximum and minimum values for each revolution of the crankshaft.
3. The pulsating flow imposes an acceleration used that adds to the net positive inlet pressure required.
4. Being more energy efficient, the positive displacement pump is normally the lead pump and the centrifugal pump is the supplement when operated in parallel.

Pertinent Centrifugal Pump Characteristics

1. The dynamic head of (or pressure rise) produced is a function of pump capacity as well as of system requirements.
2. For satisfactory operation, the flow must be kept within a limited range of the best efficiency capacity.
3. When used as a suction booster, the centrifugal pump must be designed so it cannot introduced air into the gas intolerant reciprocating pump. (Gas in positive displacement pumps, like liquids in positive displacement compressors, may cause severe hydraulic and mechanical shock).
4. Centrifugal pumps do not generate acceleration heads that impose on not positive suction head required. However, if a centrifugal pump is connected in parallel to a common suction line with a reciprocating pump, some of the system acceleration head loss from the positive displacement pump may affect the centrifugal pump.

Only two cases need to be considered: centrifugal pumps feeding:

1. In series into reciprocating pump to increase suction NPSHA to the reciprocating pump
2. In parallel to augment the delivered capacity. It would be most unusual to encounter a positive displacement pump feeding into the suction of a centrifugal pump because of the high pressure that could be imposed on the centrifugal pump suction and because of the amplification of flow pulsations resulting from interaction of the characteristic of the two pumps, which could be deleterious to both pumps.

Pump Selection and Pump Performance

There are three principal performance parameters relating to pump selection:

• Flow (or Capacity)
• Total Delivery Head
• Suction Lift

1.    CAPACITY

Required capacity, measured in flow/time is determined by one of two factors:

• If there is storage capacity it is related to total daily demand. Daily demand must first be estimated and then the hourly requirement calculated by dividing the daily demand by the number of hours the pump is required to work.
• If there is direct supply pump capacity should be related to peak hourly demand. This would be appropriate in irrigation or pressure systems.

Capacity is measured in various units including gallons per hour – gallons per minute, liters per second, liters per minute and cubic meters (1000 liters) per hour. All Davis & Shirtliff products are rated in cubic meters per hour (m3/hr)

2.    TOTAL HEAD

There are three principal components to total head of importance when specifying a pump: static head, dynamic head (friction loss) and pressure head.

Total Head = Static Head + Dynamic Head + Pressure Head

2.1  Static Head (H)

DIAGRAM 1 - Static Head Conditions

Static head is the vertical linear distance between the level of the water being pumped and either the delivery outlet or the reservoir water level, whichever is higher (see A & B). Of great

importance to note is that it is not necessarily the distance between the pump itself and the delivery point. This has particular reference to submersible pumps where the level the pump is set in the water does not determine static head. It is determined by the pumping water level (see C).

2.2  Dynamic Head

The only important component of dynamic head is pipe friction, this being determined by water velocity in the delivery pipe. The higher the velocity the higher the friction loss and it is important to match the pump to the pipeline. Friction loss values for GI and PVC pipes are given in table 1.

Some important points to note when matching pumps and pipelines are:

·  Friction losses are considerably lower in PVC pipes than GI ones. For long pipelines the use of PVC will therefore reduce pump size and energy consumed.

·  Piping can be considerably more expensive than the pumping installation and a pipe size smaller matched to a pump size larger can reduce the investment cost. Running costs will be higher though.

·  Total head reduces up the pipeline and lighter duty pipes can be used towards the system’s delivery point.

Total friction loss for a pipeline (HF) = F x L/100

Where:

F = Friction loss given for a particular flow in a specified pipe size (m per 100m pipe length).

L= Pipe length (m)

Pipe friction is not linear and increases logarithmically as velocity (or flow) increases. A typical friction loss curve is given below.

DIAGRAM 2 - Typical Friction Head Loss Curve

This diagram can be plotted using friction loss values given for a particular pipe specification at different flow rates.

2.3  Pressure Head

When delivering to an open outlet pressure at the delivery point is zero and so in most water supply installations this is not a factor in total head calculations. However, when pressure delivery is required eg. for fire installations or irrigation nozzles the required pressure at the nozzle must be included when calculating total head.

DIAGRAM 3 - Pressure Head Condition System Head Curves

In order to find the total head required on a pump, static head plus dynamic head plus friction head must be added. This can be done graphically as follows:

DIAGRAM 4 - System Head Curve

From the above graph pump 3 or pump 4 can be selected, depending upon required pump capacity.

3.    SUCTION LIFT

Centrifugal pumps have the capability of creating a vacuum in a suction pipe which enable them to suck water from below their setting level. The maximum theoretical suction lift is 1 atmosphere (approx 10m), though the maximum practical lift is well below this.

Maximum suction lift is determined by the formula:

Hmax = A – NPSH –Hf –Hv –Hs

DIAGRAM 5 - Suction Lift Conditions

Considerations relating to the various parameters are as follows:

A – Atmospheric pressure. At sea level it is 10.3m reducing by approximate 3% per 300m. Suction lift is therefore reduced at higher altitudes.

NPSH – The suction characteristic of the pump which is shown on the pump manufacturer’s curve.

DIAGRAM 6 - Typical NPSH Curve

The higher the flow the higher the NPSH and therefore the lower the available suction lift.

Hf – Friction loss in the suction pipe. This is calculated in a similar way to friction loss under section 2.2. The value increases with increasing flow thereby reducing the available suction lift.

Hv – The water vapour pressure. This is an important factor for liquids above 30oC, though is not important at normal ambient temperatures.

Vapour pressure values are as follows;-

DIAGRAM 7 - Vapor Pressure Values

HS - A safety margin, usually 1 m being acceptable. Some general points about suction conditions are as follows:

·  It is good practice to keep suction pipes as short as is practical.

·  Suction pipes must be totally airtight. If there are any leaks the pump will be unable to create the vacuum condition for suction to occur.

·  Suction pipes must be straight and laid to rise continuously to the pump. If there are any leaks in the pipe air pockets will form and the system will become air locked.

·  Suction pipes must be generously sized, one size larger than the delivery pipe being standard practice. Also all suctions should be fitted with foot valves.

·  Where the distance from the pump mounting point to the water level is greater than the available suction lift either a submersible or a jet pump should be used.

4. CENTRIFUGAL PUMP PERFORMANCE

4.1 Performance Parameters

When specifying centrifugal pumps it is important to understand the various parameters that effect pump performance and their relationship with one another.

Typically a pump curve will provide the following information.

DIAGRAM 8 - Typical Centrifugal Pump Performance Curve

Three plots are given against flow – Pressure (or Q-H curve), Efficiency (h) and Power absorbed.

Pressure: Otherwise known as the pump performance or Q – H curve and plots the pressure/flow profile of the pump.

·  At zero flow the pump will provide its maximum pressure (closed head pressure).

·  At zero head the pump will provide its maximum flow.

Efficiency (h): The efficiency curve is the plot of overall efficiency against flow. Points to note are:

·  The pump’s optimal duty point is that at which peak efficiency occurs and is usually around the mid point of the curve. The optimal performance envelop is the flow range which is greater than 90% of the pump’s maximum efficiency and applications should be within this envelope.

·  Efficiency drops considerably at high pressures and high flows and specifying a pump to operate in these sections of a curve must be avoided.

Power: The power curve is a plot of power consumed against flow. Points of note are:-

·  Maximum power consumption of a pump occurs at high flows/low pressures. Usually power consumed at high pressures is lower.

·  When coupling motors to pumps it is important to ensure that the power consumed at open delivery is less than the motor size or else motor failure may occur.

4.2 Pump Parameters

The following parameters affect pump performance:

·  Speed

·  Impeller Diameter

·  Number of Impellers

Speed: Impeller speed effects power consumed and pump performance as follows:

Speed = f (Power3)

Doubling Speed increases power consumed by a factor of 23 = 8

Speed = f(Pressure2)

Doubling Speed increases pressure by a factor of 22 = 4

Impeller Diameter: Impeller diameter effects pump performance in a similar way to speed.

Diameter = f(Power3)

A 10% increase of impeller diameter increases power consumed by (1.13 – 1) x 100 = 33%

Diameter = f(Pressure2)

A 10% increase of impeller diameter increases pressure by (1.12 – 1) x 100 = 21%

Number of Impellers

·  Adding impellers in series increases pressure though has no effect on flow. This is the effect of a multistage pump.

·  Adding impellers in parallel increases flow though has no effect on pressure. This is the effect of two pumps connected in parallel.

DIAGRAM 9 - Impeller Configurations

4.3 Pump Shaft Horse Power

Pump Shaft Horse Power can be calculated from the formula:

HP = Q x H/275 x h

Where Q = flow in m3/hr

H = Head in m

h = Pump Efficiency

Combined Displacement and Centrifugal Pumps

Flow Control in Combined Displacement - Centrifugal Pumps

Systems that involve both centrifugal and reciprocating positive displacement pumps deserve some special consideration. Centrifugal pumps are often used as suction boosters to overcome acceleration head requirements peculiar to reciprocating pumps but are rarely used to supplement flow. Some unique characteristics of each type of pump that affect the other type must be considered in the design, operation, and control or the interrelated system.

Comparison of several capacity control schemes for positive displacement pump

Control method	Degree of Modulation	First Cost	Operating Cost		Comments
Directing Acting Pumps
Steam throttle	Full zero to 100%	low		low	Steam pressure required to balance liquid piston force and overcome breakaway friction. Steam volume throttled to produce desired capacity
Power Pumps
Start-stop	Zero or 100%	low		low	Limited in frequency of starts because of temperature rise from inrush.
Multispeed motors	Steps dependent on motor winding	medium		low	Cost of motor controller switch gear motors must be assessed
Variable frequency	Full zero to 100%+	high		low	Limited by current handling capacity of solid state controller
Direct current	Full zero to 100%	High		Medium	Drive is torque speed sensitive, pump is torque pressure sensitive at all speeds. Check drive for required
Wound rotor motors	Full zero to 100%	High		low	Drive is torque speed sensitive. Pump is torque pressure sensitive at all speeds. Check drive for required torque at minimum and maximum speeds
Combustion engine	Variable for limited range	high		low	Torsional analysis required to avoid high torsional stresses.
Steam or Gas turbine	Variable for limited range	medium		medium	Drive is torque speed sensitive at all speeds. Check drive for required torque at minimum and maximum speeds.
Hydraulic torque converter	Full zero to 100%	high		high	Low full speed efficiency