10.2 On off control for pump

On off control for pump

On off control for a pump means that one or more pumps are started and stopped systematically, according to the pumping requirement. Where the flow requirement does not coincide with the pump capacities the pumps will work intermittently at a starting frequency of 1 to 15 starts per hour. In order to account for the difference between instantaneous flow requirement and the pumped flow, some form of liquid storage reservoir (storage tank) must be provided for. In the case of circulatory pumping in fully closed systems, there is no requirement for a flow storage reservoir, but there is often a requirement for storage of some other parameter, e,g, heat in a heat store.

On off control for pump is arranged automatically so that pumps are started at a high level in a storage vessel on the suction side of the pump (pump sump), or at low pressure in a storage vessel on the pressure side, in a water tower for example. Small storage Vessels operating at pressures which are greater than atmospheric pressure are called hydrophores. Start occurs at low pressure in a hydrophore and stop at high pressure.

On-off control is used to a considerable extent in pumps for water and sewage installations and for domestic drinking water installations. On-off control has a great variety of practical applications.

Certain special cases omit the liquid store, for example booster pumps (see Section 9.7 Pressure booster pump>>>) working in “dead” pipe-line networks. In order to avoid too frequent starting, an electrical interlocking device with, say, a time relay must be fitted. In certain flow ranges, pressure variations due to starting and stopping of pumps is inevitable.

Costs associated with on off control

Of the discontinuous methods of regulation, the on-off method is the most usual and provides the method with the lowest procurement cost for the pump equipment itself. However. the following additions have to be made to the pump installation costs.

• cost of storage tank or sump in which to accumulate liquid during the regulation cycle
• additional costs for starting equipment since this has to be dimensioned to cope with a very large number of starts
• additional costs for equipment to assist in the alleviation of water hammer (line shock) when stopping
• additional costs for delivery lines and fittings since these have to be dimensioned to withstand the fatigue stresses induced by water hammer.

Problems with on off control for pump when starting

The number of permitted starts per hour is a decisive factor in total procurement costs for the pump installation, but the number of starts must not on the other hand be chosen to be too high, as this may cause reduced reliability. Experience shows that the number of starts chosen can be 5 to 10 starts per hour. Thus there will be 20,000 to 50,000 starts per year and up to the time of write-off, the number of starts will amount to several millions, thus many of the component parts have to be dimensioned to withstand fatigue.

Even if the starting time of the pump is short, about 1/4 second on direct start and about 4 to 5 times longer on 3-star-delta, this can put a heavy load on the mains.

Problems with on off control for pump when stopping

A liquid flowing in a lengthy pipeline possesses considerable kinetic energy. When the liquid decelerates with the stopping of the pump, large pressure variations occur. This can be calculated and is, for short flow stopping time:

where

ΔH = change of pressure head (m)
a = speed of sound in the liquid-filled pipeline (m/s)
about 1000 m/s in steel pipe and
about 300 m/s in plastic pipe
v_o = velocity flow of liquid at the pre-stop condition (m/s)
g = 9,806 (m/s²)

In order to reduce the variations of head the flow has to be decelerated over a period of time = t_o (s), which can be roughly assessed in a pipeline of length L (m) as

t_o = 15 to 60 * (L/a)        (Equ. 10.2b)

In the case of pipelines with high points or with low resistance to fatigue, the stopping time may have to be doubled. The normal arrangements used to increase the stopping time or to reduce the stresses are:

• slow-closing valves
• air-chambers
• pumps with flywheels
• surge columns
• overpressure and underpressure valves (see Section 9.11 Water hammer protection >>>)

In all cases the flow continues for a time (t_o). To avoid the sucking of air into the pump and pipeline, a sufficient supply of liquid must be available on the suction side of the pump, whilst that same volume of liquid must not cause trouble on the delivery side through surge for example.

For air chambers, the volume of liquid is contained in the air chamber and in other cases, the storage tank or pump sump has to supply the corresponding increase in volume. When the flow requirement or inflow is negligible, which is the most in-favorable condition, the mean flow during the stop cycle can be estimated to be about 2/3 of the maximum flow. The partial storage volume, V_o (m³), between stop signal and stop flow will, for the decelerating time t_o (s) be as before:

V_o = 10 to 40 * (L/a) * Q_P

where Q_P = pump flow (m³/s)

With longer pipelines, especially for installations with slow closing valves, a large part of the operational period will be taken up by the start and stop cycles, see figure 10.2a. During these, throttle regulation takes place, see also Section 10.4 Trottle regulation using control valves, which is the worst possible method of regulation. Power consumption is determined by the relation between the times for pure on-off operation and those for valve opening and closing. Throttle regulation can increase energy consumption by anything up to 50%.

Figure 10.2a Flow variation for on-off control with long pipeline. During the start and stop periods throttle regulation takes place with slow closing valves.

Operational sequences with on-off control for pump

There can be a choice of several different operational sequences for switching in and out parallel connected units, Decisive factors to be considered are:

• the number of permitted starts per pump and per hour
• the number of permitted starts during the lifetime of the installation
• permitted variations in flow
• costs of storage tanks or pump sumps

The differences between various possible sequences, designated A, B and C, is shown in the following three schematic examples applying to an installation with two identical parallel connected pumps Pl and P2. It becomes clearest if the condition is examined when the flow requirement, inflow, is equal to 1,5 times the flow of one pump.

Operational Sequence A, Figure 10.2b

P1 operates continuously and P2 operates with on-off. The cycle time t_A is obtained when dimensioning for P2 s storage or partial sump volume. In this operational sequence there are moderate flow variations.

Figure 10.2b Flow variation and operating times for sequence A

Operational Sequence B, Figure 10.2c

P1 starts first, and then, at an increased level, so does P2. Stop occurs simultaneously for both pumps. The flow variation is very wide, but the storage or partial sump volume for P2 is reduced to almost half of that for Sequence A. Because of the common stop, one sensor signal is saved. Operational Sequence B has been applied to about 90% of all sewage pump installations up to the present time.

Figure 10.2c Flow variations and operating times for Sequence B.

Operational Sequence C, Figure 10.2d

Here we get the same flow variation as in operational Sequence A, except that the stop sequence is the same as the start sequence, i.e. the stop signal always stops the pump which started first. Operational Sequence C means a doubling of the cycle time, i.e. t_c = 2t_A, meaning the halving of the storage or partial sump volume for P2.

Figure 10.2d Flow variation and operating times for Sequence C.

Where there are more than 2 pumps, the same principles as above are used, as also with the switching over of pole-changing motors, for example. For a number of pumps working in parallel, the partial storage volume is then determined for each pump depending upon its respective incremental flow Q₁, Q₂, Q₃, etc, as obtained from the pump and system curves, according to figure 10.2e.

Figure 10.2e Flow increments Q₁, Q₂, and Q₃ obtained by switching in various numbers of parallel-connected pumps.