Reciprocating Pumps: Parts, Diagram, Types and Working Principle!
Introduction to Reciprocating Pumps:
When a plunger is adopted, the liquid acts on only one side of the piston and such a pump is called a single acting pump. In such a pump the liquid is sucked into the cylinder during the outward stroke and is forced out during the inward stroke. If in a pump the liquid acts on both sides of the piston, then it will suck as well as deliver during each of the outward and inward strokes and such a pump is called a double acting pump.
Fig. 25.1 shows a diagrammatic view of a single acting reciprocating pump.
The pump consists of the following components:
(i) The cylinder C.
(ii) Plunger or piston P.
(iii) Suction pipe S with a one-way control valve V1 which can admit the flow from the suction pipe into the cylinder.
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(iv) Delivery pipe D with a one-way control valve V2 which can admit the flow from the cylinder into the delivery pipe.
(v) Connecting rod.
(vi) Crank.
The rotation of the crank brings about an outward and inward movement of the plunger P in the cylinder C. During the suction stroke of the pump, the plunger moves outwards causing a vacuum in the cylinder. The pressure of the atmosphere acting on the sump water surface forces the water up the suction pipe, forcing open the suction valve V1, and the water enters the cylinder.
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During the inward stroke of the plunger the high positive pressure created in the cylinder, closes the suction valve V1 and opens the delivery valve V2, and the water collected in the cylinder is forced up the delivery pipe.
Theoretically, for one revolution of the crank the quantity of water raised up the delivery pipe is equal to the stroke volume of the cylinder, in a single acting pump, and twice this volume in a double acting pump.
The slip is in most cases positive. However in some special circumstances, the actual average discharge will be slightly greater than the theoretical average discharge. In such a case the slip of the pump will be negative.
The Ideal Indicator Diagram:
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This is a diagram showing the pressure head in the cylinder during the suction and delivery strokes of the pump under ideal conditions. This diagram shows the pressure head in the cylinder plotted as the vertical ordinate, while the stroke length is represented by the horizontal ordinate.
Fig. 25.2 shows the ideal indicator diagram. The absolute pressure heads are measured above a reference horizontal line O—O. The line ef corresponds to the atmospheric pressure head. The line ab corresponds to the pressure head in the cylinder during the suction stroke.
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We know during the suction stroke the absolute pressure head in the cylinder = Hat – Hs. The line cd corresponds to the pressure head in the cylinder during the delivery stroke. We know during the delivery stroke the absolute pressure head in the cylinder = Hat + Hd.
The Double Acting Pump:
The double acting pump works on both sides of the piston. In this arrangement the piston is connected to a piston rod whose other end is connected to the connecting rod. Ignoring the size of the piston rod the volume of water delivered in one revolution of the crank = 2AL. Hence the average discharge of the pump in this case,
Pressure Variation due to Acceleration of Piston:
Due to the reciprocating motion of the plunger or piston, it will have an acceleration during the first half of a stroke and a retardation in the latter half of the stroke. This will transmit corresponding acceleration and retardation to the water in the suction and delivery pipes, and due to the inertia of this water a pressure variation occurs in the cylinder.
Let us assume that the length of the connecting rod is very large compared with the crank radius so that we can consider the motion of the plunger or piston as simple harmonic.
Let ω = angular velocity of the crank. Let the angular displacement of the crank say from the commencement of the suction stroke be θ in an interval t.
Effect of the Acceleration in Suction Pipe on the Pressure Head in the Cylinder:
Consider the pressure head in the cylinder during the suction stroke. When the piston moves, it has to produce a negative or vacuum pressure to lift the water to a height Hs and also to accelerate it. We know the pressure head needed to produce the acceleration is –
It can be noted that the minimum pressure head in the cylinder occurs at the beginning of the suction stroke. The indicator diagram for suction stroke corrected for acceleration of water in the suction pipe is shown in Fig. 25.5. The ideal indicator diagram abfe is modified to a1b1fe.
It is important to realize that as the speed of the pump is increased, ha max also increases and for a certain critical value of the speed the absolute pressure head at the beginning of the suction stroke reaches the separation pressure head Hsep. Thus, there exists a maximum limit for the speed of the pump.
Corresponding to the maximum speed Hat – Hs – ha max = Hsep.
From this condition we can determine ha max and the corresponding maximum speed of the pump.
It may further be noted that due to acceleration of water, only the shape of the indicator diagram is changed but its area is not changed, i.e., for a given speed of the pump the work done by the pump is not affected due to acceleration of water in the pipe.
Effect of Acceleration in the Delivery Pipe on the Pressure Head in the Cylinder:
Consider the pressure head in the cylinder during the delivery stroke. When the piston moves it has to produce a pressure to sustain the water column of height Hd and also to accelerate it. We know the pressure head needed to produce the acceleration is –
It can be noted that the minimum pressure head in the cylinder during the delivery stroke occurs at the end of the stroke.
The indicator diagram for the delivery stroke corrected for acceleration of water in the delivery pipe is shown in Fig. 25.6. The ideal indicator diagram cdef is modified to c1d1ef.
It is important to realize that as the speed of the pump is increased ha max also increased and for a certain value of the speed the absolute pressure head at the end of the delivery stroke reaches the separation pressure head Hsep. When separation occurs, dissolved gases in the water are liberated resulting in the discontinuity of the flow. This also fixes a limit for the speed of the pump. Corresponding to this maximum speed.
Hat + Hd – ha max = Hsep
From this condition we can determine ha max and the corresponding maximum speed of the pump.
It may further be noted that due to acceleration of water, only the shape of the indicator diagram is changed but its area is not changed i.e., for a given speed of the pump the work done by the pump is not affected due to acceleration of water in the pipe.
Delivery Pipe with a 90° Bend:
Suppose the delivery pipe consists of a vertical part and a horizontal part forming a 90° bend.
Fig. 25.7 (a) shows a delivery pipe which first runs vertically and then horizontally. Fig 25.7 (b) shows a delivery pipe which first runs horizontally and then vertically. Let in both the cases the length of pipe and the vertical height be the same. In both the cases, the condition at J will be the same, since in both the cases Hd has same value.
Considering the arrangement in Fig. 25.7 (a), there is a possibility of separation to occur at K earlier than at J. This is because at K, Hd = 0 and there is still a considerable length of pipe KE beyond the elbow K.
If the Length KE = ld’ for the condition of separation at K the end of the delivery stroke.
Absolute pressure head at K.
This is the condition for determining the limiting speed for separation to occur during the delivery stroke. In the arrangement shown in Fig. 25.7 (b), when separation occurs, it does at J rather than at K and at this stage,
Absolute pressure head at J –
Hence the arrangement shown in Fig. 25.7 (b) is better than the arrangement shown in Fig. 25.7 (a). In the arrangement shown in Fig. 25.7 (b) the pump can run at a higher speed than in the other arrangement.
Effect of Pipe Friction on the Pressure Head in the Cylinder:
As the water flows through the suction and delivery pipes, there will be a frictional resistance to its motion. At any instant if the velocity of water in the pipe is v the head lost due to friction –
The indicator diagram corrected for acceleration of water in the pipes and pipe friction is shown in Fig. 25.8. The area of the indicator diagram is increased due to pipe friction indicating that the work done by the pump is increased to overcome the frictional resistance.
The loss of head due to friction varies parabolically from zero at the beginning of a stroke to a maximum value at the middle of the stroke and finally reaches the zero value at the end of the stroke.
Negative Slip:
The slip of a pump is said to be negative when the actual average discharge of the pump is greater than the theoretical average discharge of the pump.
Consider a single acting pump. We know the absolute pressure head in cylinder at the end of the suction stroke.
Pump with Air Vessels:
The ordinary reciprocating pump discussed so far has certain shortcomings.
These are:
(i) The discharge is not uniform. Particularly in the single acting pump the discharge is intermittent.
(ii) The pump cannot be made to run at high speed. There is a fear of separation to occur at the commencement of the suction stroke or near the close of the delivery stroke. Thus the discharging capacity of the pump is very much limited.
(iii) Considerable amount of work is done in overcoming the frictional resistance in the suction and delivery pipes.
To rectify the above shortcomings of the pump, air vessels are fitted to the suction and delivery pipes close to the cylinder as shown in Fig. 25.9.
We know the acceleration pressure head depends on the length of the accelerated column of water. By fitting air vessels to the suction and delivery pipes close to the cylinder, the length of the accelerated column of water is reduced. For instance, let us consider the air vessel fitted to the delivery pipe.
The air vessel consists of an inverted cast iron chamber with the opening at its lower end through which the water can flow. As the water level in the air vessel rises, the air trapped in the upper region gets compressed and this compressed air will force out the water when the pressure of the water falls.
With this arrangement, the flow in the delivery pipe beyond the air vessel becomes practically uniform. During the middle part of the delivery stroke, when the piston is pushing the water into the delivery pipe with a velocity greater than the mean velocity, the additional discharge will flow into the air vessel. Near the end of the stroke when the water is delivered into the delivery pipe with a velocity less than the mean velocity, the water will flow out of the air vessel and make up the deficiency.
Thus, the flow rate beyond the air vessel is practically uniform. The only quantity of water which gets accelerated is the quantity in the delivery pipe between the cylinder and the air vessel. Hence the acceleration pressure head can be reduced to a very small or ignorable value by providing the air vessel very close to the cylinder.
Similarly, by providing an air vessel to the suction pipe close to the cylinder, there will be practically no acceleration pressure head during the suction stroke also, and the flow becomes practically uniform in the part of the suction pipe between the sump and the air vessel.
As the water flows in and out of the air vessel, the pressure of the air will vary. By providing the air vessel of large size compared to that of the pipe, the variation of the air pressure can be reduced. For analysis purposes let us assume that the size of the air vessel is so large that any change in water level in it may be ignored. This amounts to the assumption that the pressure of air in the air vessel is taken to remain constant.
Multicylinder Pumps:
These are pumps with more than one cylinder, the most common of them are the two-throw and the three-throw pumps.
(i) The Double Cylinder Pump or the Two Throw Pump:
This pump consists of two single acting cylinders. Each cylinder is provided with a suction and a delivery pipe with their valves and separate piston or plunger.
Fig. 25.11 shows the cylinder and the plungers of the two throw pump. The two plungers are driven simultaneously by cranks which are set at 180°.
The average discharge of the pump is 2(ALN/60). This is similar in principle to the double acting pump.
(ii) The Triple Cylinder or the Three Throw Pump:
This pump consists of three single acting cylinders. Each cylinder is provided with a suction and delivery pipe with their valves and a separate piston or plunger.
The three plungers are driven simultaneously by cranks which are set at 120°.
The average discharge of the pump = 3 (ALN/60).
Rate of Delivery of Reciprocating Pumps:
Due to the simple harmonic motion of the piston caused by the crank and connecting rod mechanism, the velocity of water in the pipes is subjected to corresponding variations. At any instant, the velocity of water in the pipe is equal to (A/a) ωr sin θ. Hence the instantaneous discharge in the pipe = q = A ωr sin θ. Thus θ is a function of θ and is, therefore, different for different values of the crank angle.
We will now consider the various types of reciprocating pumps:
(i) Single Cylinder, Single Acting Reciprocating Pump:
Fig. 25.13 (a) shows a plot of rate of delivery against crank angle θ. We know in the first half revolution of the crank the water is only sucked into the cylinder and during the second half of the revolution this water is discharged through the delivery pipe. The cycle is repeated.
(ii) Single Cylinder Double Acting Pump:
In this case we know, each stroke is a suction as well as a delivery stroke. While a suction stroke is going on one side of the cylinder, a delivery stroke is going on the other side of the cylinder. See Fig. 25.13 (b).
(iii) Two Throw Pump:
In this case, there are two cylinders. Each cylinder is provided with a suction and delivery pipe with their valves and a separate piston. The pistons are driven simultaneously by cranks which are set at 180°. The discharge crank angle plot is similar to that of the ordinary double acting pump.
(iv) Three Throw Pump:
In this case, there are three cylinders. Each cylinder is provided with a suction and delivery pipe with their valves and a separate piston. The pistons are driven simultaneously by cranks which are set at 120°. The discharge-crank angle plot is shown in Fig. 25.14.
A comparison of merits and demerits of reciprocating pumps over centrifugal pumps is given in the table below:
