For pumping of sewage following types of pumps are used: 1. Centrifugal Pumps 2. Reciprocating Pumps 3. Air Pressure Pumps or Pneumatic Ejectors.
These are briefly described below:
Type # 1. Centrifugal Pumps:
Centrifugal pumps are most commonly used for pumping sewage, because these pumps can be easily installed in pits and sumps, and can easily transport the suspended matter present in the sewage. A centrifugal pump consists of a revolving wheel called impeller which is enclosed in an air tight casing to which suction pipe and delivery pipe or rising main are connected.
The impellers of centrifugal pumps have backward curved vanes which are either open or have shrouds. Open impellers have no shrouds. Semi-open impellers have only a back shroud. Closed impellers have both the front and the back shrouds. For pumping sewage either open or semi-open type impellers are commonly used.
The clearance between the vanes of the impeller is kept large enough to allow any solid matter entering the pump to pass out with the liquid so that the pump does not get clogged. As such for handling sewage with large-size solids, the impellers are usually designed with fewer vanes. The pumps with fewer vanes in the impeller or having large clearance between the vanes are called non-clog pumps. However, pumps with fewer vanes in the impeller are less efficient.
A spiral shaped casing called volute casing is provided around the impeller. At the inlet to the pump at the centre of the casing a suction pipe is connected, the lower end of which dips into the liquid in the tank or sump from which the liquid is to be pumped or lifted up.
At the outlet of the pump a delivery pipe or rising main is connected which delivers the liquid to the required height. Just near the outlet of the pump on the delivery pipe or rising main a delivery valve is provided. A delivery valve is a sluice valve or gate valve which is provided in order to control the flow of liquid from the pump into the delivery pipe or rising main.
The impeller is mounted on a shaft which may have its axis either horizontal or vertical. The shaft is coupled to an external source of energy (usually an electric motor) which imparts the required energy to the impeller thereby making it to rotate. When the impeller rotates in the casing full of liquid to be pumped, a forced vortex is produced which imparts a centrifugal head to the liquid and thus results in an increase of pressure throughout the liquid mass.
At the centre of the impeller (which is commonly known as eye of the impeller) due to the centrifugal action a partial vacuum is created. This causes the liquid from the sump, which is at atmospheric pressure, to rush through the suction pipe to the eye of the impeller thereby replacing the liquid which is being discharged from the entire circumference of the impeller. The high pressure of the liquid leaving the impeller is utilized in lifting the liquid to the required height.
Pumps for sewage pumping are generally of all cast iron construction. If the sewage is corrosive then the stainless steel construction may have to be adopted. Also, where the sewage would contain abrasive solids, the pumps constructed of abrasion-resistant material or with elastomer lining may be used.
Computation of the Total Head of Pumping:
The total head of pumping consists of the following components:
(i) Static head Hs, which is the difference between the level of liquid in the suction sump i.e., the wet well and the level of liquid in the high level sewer to which the sewage is delivered by the pump.
(ii) Velocity head (V2/2g) at the point of discharge.
(iii) Head loss HL, which includes the head loss due to friction in suction and delivery pipes as well as head loss in valves, bends and all such appurtenance both on the suction and delivery sides.
Thus the total head against which the pump has to operate is given by-
With the pump running, if the discharge of the pump is more than the inflow, the level of the liquid in the suction sump would keep falling. By this the suction head component in the total head would keep increasing. Reverse will be the case when the inflow is more than the discharge by the pump.
By varying the size of the suction and delivery pipes in a pumping system the velocity of flow would vary which in turn would cause a change in the velocity head which varies as the square of the velocity of flow. This also causes a variation in the frictional losses because these also vary as the square of the velocity of flow or flow rate.
Thus the total head of pumping varies with the flow rate or discharge of the pump. At the stage of planning it is therefore necessary to compute the total head of pumping over a range of flow rates or discharges, for different variations in the levels of liquid in the suction sump and for different options of piping sizes and layouts.
The values of the total heads of pumping so computed are plotted against the flow rates or discharges. A plot of total head of pumping versus flow rate or discharge is known as system-head curve for the pumping system. Fig. 7.4 shows a typical system-head curve for a pumping system with a given static head and a given piping size and layout.
With an increase only in the static head the new system-head curve will be a curve shifted parallel upwards as shown in Fig. 7.5. Further for a smaller size of piping the parabolic portion in the system-head curve will be steeper as shown in Fig. 7.6.
From the system-head curves for a pumping system with variations in static levels and different piping sizes and layouts, one can know what the total head would be for the most average operating conditions, which then can be specified as the total head of pumping.
Operating Point or Operating Range of a Centrifugal Pump:
The operating point of a centrifugal pump is the point of intersection of the system-head curve with the head-discharge (H Vs Q) characteristic curve of the pump. Thus for obtaining the operating point the system-head curve and the head-discharge characteristic curve of the pump are plotted on the same sheet as shown in Fig. 7.7.
The head-discharge characteristic curve of a centrifugal pump is a drooping parabola with the pump discharge being less when the head is more. When the pump is put into a system it meets the head as demanded by the system, which is as per the system-head curve.
At the point of intersection P of the system-head curve with the head- discharge characteristic curve of the pump, since the head demanded by the system for a certain flow rate or discharge is equal to the operating head of the pump at the same flow rate or discharge, point P represents the operating point of the pump.
The system-head curve of a pumping system will change due to any change made in the system. If the system-head curve changes the operating point will shift. The most significant change that may occur in a pumping system is due to variation in the level of liquid in the suction sump.
There will be different system-head curves corresponding to low water level (L.W.L.) and high water level (H.W.L.) in the suction sump as shown in Fig. 7.8. Thus if the level of liquid in the suction sump depletes during pumping from H.W.L. to L.W.L, the operating point of the pump would vary from a low-head and high- discharge point P1 to high head and low-discharge point P2 as shown in Fig. 7.8. The range between points P1 and P2 is known as the operating range of the pump due to fluctuations in the level of liquid in the suction sump.
Selection of Pumping Unit:
With a given or known system-head curve it is required to select a pump to deliver the anticipated flow. For this the system-head curve and the head-discharge characteristic curve of the pump are plotted on the same sheet and the operating point is found (see Fig. 7.7). The operating point P gives the head and discharge or flow rate at which the pump will be operating. As such the pump selected should be such that the operating point is as close as possible to the best efficiency point.
Operation of Pumps in Parallel:
When two or more pumps are lifting liquid from a common sump and discharging into a common closed conduit or header, the pumps are said to be running in parallel. The performance characteristics of the pumps running in parallel suffer mutual influences. In this case the flow obtained in the header is what is contributed by all the running pumps together.
The combined characteristic curve of the pumps running in parallel is obtained by reading against different heads, the values of the discharge Q obtained front the characteristic curves of individual pumps and plotting the addition of the Q values against the respective heads, as shown in Fig. 7.9.
The operating point of pumps in parallel operation is the point of intersection of the combined head-discharge characteristic curve with the system-head curve. The point of intersection on the combined head-discharge characteristic curve is at a head higher than that at the point of intersection on the head-discharge characteristic curve of a single pump and hence the discharge of each pump at the operating point gets reduced.
Thus as shown in Fig. 7.9 when only one pump is operating, the discharge at the operating point is Q1 but when two identical pumps are operated in parallel the point of operation gets shifted upwards and the discharge of each pump at the new operating point becomes Q2 such that Q2 <Q1.
Similarly when three identical pumps are operated in parallel the discharge of each pump at the new operating point becomes Q3, such that Q3 < Q2 < Q1. From this it is clear that two identical pumps put into parallel operation will give discharge less than double the discharge of only one pump operating.
It means that to double the discharge capacity of pumping, it is not adequate to commission two identical pumps in parallel operation. For this one must study what combination of pumps with different head-discharge characteristic curves can give such a combined characteristic curve which will have its intersection with the system-head curve at a point having discharge equal to the desired double discharge.
As shown in Fig. 7.9, if there are two identical pumps running in parallel, individual pump would be contributing a discharge Q2. If one of the pumps would trip the system would have only one pump running and giving a discharge which is more than Q2. At higher discharge, the pump would draw more power, which should not overload its motor.
As such while putting the pumps into parallel operation the discharge Q2 of individual pump in parallel operation should be such that it is somewhat to the left of the discharge at the best efficiency point (b.e.p.) of the pump, so that in the event of tripping of any other pump/s the higher discharge such as of the running pump will be nearer to its best efficiency point (b.e.p.).
In the case of high head pumping multi-stage pumps are used. A multi-stage pump consists of two or more identical impellers mounted on the same shaft, and enclosed in the same casing. All the impellers are connected in series, so that liquid discharged with increased pressure from one impeller passes through the connecting passages to the inlet of the next impeller and so on, till the discharge from the last impeller passes into the delivery pipe.
The impellers are surrounded by guide vanes or diffuser meant for the recuperation of the kinetic energy of the liquid leaving the impeller into pressure energy. According to the number of impellers fitted in the casing a multi-stage pump is designated as two-stage, three-stage, etc.
For a multi-stage pump the value of head H to be used for computing its specific speed (equation 7.1) is obtained by dividing the total head developed by the number of stages. Thus by making the head to be shared by more than one impeller the specific speed for each impeller will be better.
Cavitation in Centrifugal Pumps:
In a centrifugal pump the flow must reach the eye of the impeller with such absolute pressure head that it will be higher than the vapour pressure and the net positive suction head required (NPSHr) by the pump.
The absolute pressure head of the flow, as it reaches the eye of the impeller can be found by deducting from the pressure on the liquid in the suction sump, which is atmospheric in the case of an open sump such as the wet well, firstly the static head between the liquid level in the suction sump and the centre line of the pump, if the pump’s centre line is above the liquid level, i.e., if there is a suction lift.
If the centre line of the pump is below the liquid level, i.e., if the suction is flooded, the static head will have to be added and not deducted. Next, the velocity head appropriate to the suction size will have to be deducted. Also the frictional losses up to the eye of the impeller will also have to be deducted. Besides this the flow has also to overcome the losses of head due to shocks, twists, turns and turbulences at the eye of the impeller.
The energy (head) required to overcome these head losses caused in the suction passage of the pump and the inlet edge of the impeller vanes is called the net positive suction head required (NPSHr) of the pump. So the positive absolute pressure of the flow, as it reaches the eye of the impeller should be more than the vapour pressure pv even after providing for (NPSHr).
The net positive suction head required (NPSHr) for a pump is given by the manufacturer of the pump. If NPSHa is not greater than NPSHr, vapour bubbles get formed, which while travelling along the flow, being compressible receive energy from the impeller which builds up the pressure inside them and the resultant compression reduces their volume culminating in the collapse of the bubbles with sudden release of the energy.
This causes impact and vibrations. All this phenomenon is called cavitation. Cavitation can cause very serious damages. In order to avoid cavitation it is to be ensured that NPSHa is always more than NPSHr. The formula given above for NPSHa indicates that it is possible to control the value of NPSHa by altering the physical arrangement of the pump installation such as by changing the size or the arrangement of the suction pipe, or by changing the suction lift.
Power and Efficiency of Centrifugal Pump:
If Q is pump discharge (or rate of lifting of liquid by the pump) in m3/s (or cumec) and w is specific weight of liquid in kN/m3, then the weight of liquid W lifted by the pump per second is given by the expression-
In metric units the input power required to drive the pump is given by the expression-
Type # 2. Reciprocating Pumps:
Reciprocating pumps are much less employed these days for sewage pumping, because of their high initial cost, difficulty in maintenance and greater wear and tear of valves. However, in cases where it is required to deal with difficult sludges and where large quantity of sewage is to be pumped against low heads, reciprocating pumps may be used after passing the sewage through screen with 20 mm spacing.
Reciprocating pumps are usually of two types:
(1) Ram type and
(2) Propeller type.
In the ram type reciprocating pump, a piston or plunger moves through glands displacing liquid in a vessel. The piston or plunger is not arranged to fit closely in the cylinder. The diaphragm pump is an example of the ram type reciprocating pump which is commonly used.
In the propeller type reciprocating pump a multiple blade screw rotor or propeller moves vertically inside a pump-casing causing the sewage to be lifted. It draws liquid through inlet guide vanes and discharges through outlet guide vanes. Thus its action is somewhat similar to that of a ship’s propeller. The axial-flow screw pump is an example of the propeller type reciprocating pump.
Diaphragm pump is a ram type reciprocating pump. As shown in Fig. 7.10 a piston or plunger is attached to the centre of a circular rubber diaphragm, the outer edge of which is bolted to a flange on the pump. The flexibility of the diaphragm permits the up and down motion of the plunger thereby increasing or decreasing the capacity of the pump-casing.
During upward movement of the plunger, liquid flows into the pump through the suction valve, while downward movement of the plunger closes the suction valve, and forces the liquid through the delivery valve (provided in the plunger) out to discharge. The diaphragm pump is simple, durable and needs no priming. However, after some use, the rubber diaphragm wears out needing replacement.
Type # 3. Air Pressure Pumps or Pneumatic Ejectors:
Pneumatic ejectors are used for pumping or lifting small quantities of sewage.
The conditions favouring installation of pneumatic ejectors are-
(i) Where small quantity of sewage is to be lifted from cellar or basement of a building to a high level sewer;
(ii) Where the quantity of sewage from a low lying area does not justify the construction of a pumping station; and
(iii) Where a centrifugal pump of small capacity is likely to clog.
Pneumatic ejectors use compressed air for lifting sewage. Fig. 7.11 shows Shone’s pneumatic ejector which is commonly used in practice. It consists of an air tight cast iron chamber with a spindle having two cups-upper cup D which is inverted and lower cup C. Two reflux valves (or check valves) V1 and V2 are provided at the inlet and the outlet points respectively.
A compressed air inlet valve V3, is provided which is operated by a lever arrangement with a counter weight as shown in the figure. Compressed air is supplied through this valve at a pressure of about 0.15 N/mm2 (1.5 kg (f)/cm2). The air in the chamber can escape through the exhaust.
The sewage flowing under gravity enters the chamber through the inlet valve K, and rises slowly in the chamber, the outlet valve V2 and the compressed air inlet valve V3 being closed at this stage. As the sewage level rises the air from the chamber escapes through the exhaust.
When the sewage level reaches the rim of the upper inverted cup D the air inside this cup is entrapped. Further rise in the sewage level in the chamber makes the entrapped air to exert vertical pressure on the inner bottom surface of the upper inverted cup D. Due to this the cup D is lifted up and through the lever arrangement the compressed air inlet valve F3 gets opened and at the same time the exhaust gets closed.
The air under pressure entering the chamber from valve V3 forces the sewage inside the chamber to flow through the outlet valve V2 into the outlet pipe which carries it to a high level sewer. At this stage when the outlet valve V2 and the compressed air inlet valve F3 are open, the inlet valve V1 is closed.
The discharge of the sewage from the chamber continues till the sewage level in the chamber falls to such a point that the weight of the lower cup C and the sewage it contains causes the cup C to drop. The lower cup C and the upper inverted cup D being connected by one rod, when the cup C drops the cup D also drops and at the same time the compressed air inlet vale V3 gets closed and the exhaust gets opened.
The sewage then starts entering the chamber through the inlet valve V1 as before and the process is repeated. The outlet valve V2 opens in one direction only and therefore the back flow of sewage from the high level sewer into the chamber of the ejector is prevented. Further while the ejector is discharging the inlet valve V1, remains closed and the incoming sewage is retained above the inlet valve or it is directed towards another ejector.
To obtain nearly uniform rate of sewage flow, the ejectors are usually installed in pairs so that when one is filling the other is discharging.
The merits of pneumatic ejectors being that they have no clogging parts and they work silently with the compressed air easily supplied from a central station. These may be employed economically for a maximum lift of about 6 m or so. They also avoid the necessity of installing screens and underground suction wells. Their capacities are, however, small varying from 500 to 10 000 litres.
The principal demerit of pneumatic ejectors is that they have very low efficiency seldom above 15 per cent except when working against low heads.