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Working and construction of Rotary reciprocating pumps
These pumps as name implies, have a rotary as well as reciprocating motion. They are also known as uneven delivery pumps because the stroke of the pumps can be varied and as such can be made to carry dissimilar quantities of liquid as and when required. In this article detail of rotary reciprocating pump has been discussed.
First of all I want to tell you that rotary pump have rotor to do the whole work of pumping. Have you ever seen the pump in your household, which is used to pump water? Well that pump is a centrifugal pump which comes in this category only. These are very handy and require very less maintenance if run properly. Now let us discuss it further.
Types of variable delivery pumps
1)The radial piston pumps
"Hele-Shaw pump" where the movement of the pistons relative to the cylinders is radial.
2) The axial piston pumps
"Swash-plate pump" where the movement of the pistons relative to the cylinders is axial. Constant-speed variable-flow rotary pumps are used as power units in hydraulic control systems, eg. ship steering gears.
Principle of rotary reciprocating pump
The principle of operation of the 'Hele-Shaw' pump is as following:
1)In the no-flow position
The innermost body and the bearing locating the path of the piston rod ends are co-axial. This means that the whole system rotates jointly, with no relative reciprocating motion of the pistons in the cylinder.
2) In the flow position
The outer bearing is tangentially displaced by a control rod,therefore the centers of the central body and the bearing centre are no longer co-axial. This means that the piston rod ends are forced to follow the outer bearing as they rotate, which means that a relative reciprocating motion of the pistons occurs within the cylinders. We can see this as the working of pistons in internal combustion engine. A particular piston making one revolution withdraws oil through the suction port in the first half-revolution and discharges oil through the discharge port during the next half-revolution .This is same like we play with water cannon in holi. The rate of discharge is controlled by the axial displacement of the outer bearing to the bearing of the central body, which can be determined by the control rod. The control rod is provided by the stuffing box to seal the fluid coming out of the pump. This pump produces very high pressure and hence are used where there is a demand of the high pressure.
Let us discuss the pump in details:
1) Reciprocating Steam Pump
In some areas in place of using electric motors we steam as driver for pump. This type of pump found application in petrol or other combustible fluid. There are numerous forms of such pumps, both horizontal and vertical, used for all duties on Industries. The reciprocating motion can be through connecting rod from electric motor drives or other forms, but still one of the most common forms is the direct steam drive for fuel oil transfer ,to reduce the chances of fire and other problems.
2)The Weir Steam Pump
In the weir steam pump ,the steam end consists of a forged steel piston with rings of cast iron with a vertical lateral clearance for the plain rings of about 70 pm and a butt gap clearance of about 0 pm when fitted. The cast iron steam cylinder has steam top and bottom from a bolted on valve chest which is provided with the required steam and exhaust valves for drain cocks. A lubricator for hand use is usually provided, filled with cylinder oil graphite, but for superheated steam mechanical lubricators are usually used because otherwise the lubricator will evaporate. A mechanical stroke counter is also sometimes fitted, to count the number of stroke covered by the piston. The steam piston is bigger in diameter than the bucket (ordinary piston) to allow the pump to work at lower steam pressures than discharge feed pressure.
As numerous sizes rump are used for the given sizes and clearances. Generally 150 mm bore pump, bigger pumps having proportionately larger clearances are used.A nickel steel ,piston rod and a brass or bronze bucket rod connect by screwing into a main steel crosshead and are locked by a steel taper pin. The full stroke of the pump must be utilized as short stroking produces ridges in the working bores. To adjust the strokes the valve spindle is screwed up until the piston is striking the top cover and then screwed down and locked to allow the piston to approach to within say 12 mm of the end cover. Some clearance is always given for the piston so that in case of vibration it does not touches the upper or lower part and hence damage the pump. The process is repeated using the bottom nut and lock nut on the threaded spindle for the cylinder bottom. The water end is of cast iron with a gunmetal bucket working in a brass or bronze liner (cast iron throughout is used for oil pumps).The reason of using the different metal is that water sometime is corrosive in nature.
The bucket usually has two grooves into which are fitted special ebonite (or tufnol) rings, the lateral clearance being about 220 micrometer and the butt gap clearance about 800 micrometer. The basic function of the rings is to seal the space so that there is no leakage during suction and discharge. The rings are cut and then heated in boiling water to make them flexible, the butt gap being adjusted by trying in the liner bore. The cuttings of the ring is also special and are cut according to the needed pressure. The double acting chamber has a twin unit valve chest at the front, each unit, one top and one bottom, having a suction and discharge valve set. The valves are spring loaded for automatic closing and avoiding uneven opening. Such valves are small circular valves, about five or seven in number, in a circular pattern, the valves being spring loaded from the guard plate. For heavy duty, say for example hot feed water, etc,, the valves and seats are of stainless steel and are of the flat faced type. Each valvechest is usually provided with a small sentinel type relief valve on the top covers.The relief valve is fitted for safety purposes. The pump is also fitted with air pet cocks, drain plugs, air vessels, float control devices, suction and discharge valve.
3)The Weir Type Valve Gear pump
The valve spindle (which is a cylindrical rod)driving rod is connected to a flat plain outside steam slide valve which works on, and is carefully bedded to, the flat back face of a round shuttle valve which distributes cylinder steam and exhaust . The slide valve or auxiliary valve has a vertical motion and the shuttle valve has an axial motion. That means we are having two different motions in two different plane. The shuttle valve works at its ends inside hollow bells, the bells being a smooth sliding fit over the shuttle. This type of pump are very limited in use because of its complicated working nature.Since the moving parts in this type of pump is more so it requires more maintenance and hence it not that efficient then the other pump. One thing is to be noted that any pump which is using steam for their move have to be checked regularly so that there is no corrosion in these costly setup
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