POSITIVE DISPLACEMENT PUMP DESIGN AND CONSTRUCTION
There are two general classifications for Positive Displacement pumps:
Reciprocating Pumps operate with a back-and-forth motion, and consequently have a pulsating or fluctuating discharge. Rotary Pumps provide a smoother flow than reciprocating pumps.
6.1 Reciprocating Pump
Reciprocating pumps are classified by the number and arrangement of cylinders; whether the liquid end has a plunger or piston; whether it is single or double acting and the type of driver. The number of combinations of each is almost endless. This unit is confined to the types which are most frequently used in oilfield services. Before continuing, it will be helpful to define a few terms that will be used to describe reciprocating pumps:
|Stroke||Movement of a piston from one end of the cylinder to the other end.|
|Inboard Stroke||Movement of the piston toward the drive end of the pump.|
|Outboard Stroke||Movement of the piston away from the drive end of the pump.|
|Full Stroke||Movement of the piston from one end of the cylinder to the other end and back to its original position.|
|Single Acting||Pump which discharges liquid during one half of a full stroke. Liquid enters the cylinder during one stroke of the piston and discharges from the cylinder during the reverse stroke of the piston.|
|Double Acting||Pump which discharges liquid on each stroke of the piston. Most oilfield pumps are double acting.|
|Simplex||Single cylinder pump|
|Duplex||Dual cylinder pump|
|Triplex||Triple cylinder pump|
|Multiplex||Any reciprocating pump with more than one cylinder.|
6.2 Power Driven Piston Pumps
Power provided by a motor or engine in a rotary motion can be changed into reciprocating motion in the same mechanical manner that an automobile engine changes reciprocating power strokes into rotary movement of the crankshaft. This action is reversed in a power driven reciprocating pump.
The electric motor or engine gives rotary motion to a crankshaft. Each throw on this crankshaft is mechanically linked to the reciprocating end by connecting rods, and crossheads. See Figure 37. One significant advantage of the rotary powered unit is that several parallel liquid end cylinders can be operated easily. The most common multi-cylinder pumps have 2, 3 or 5 cylinders. Pistons are used for powered pumps in lower pressure services. To obtain higher pressure, another type of pump end – a plunger – is used. Each cylinder has its own rod, packing, piston and suction/discharge valve set.
POWER DRIVER COMPONENTS
In the motor driven piston pump, the pumping rates may be varied either by changing the piston speed or length of the stroke.
The frame or crankcase of the power end usually is a one piece high strength steel casting. A cover plate allows access to the internal parts (the crankshaft, bearing, gears, connecting rods, and inserts) so they can be maintained without disturbing the pump end. Seals (packing) stop leakage out of the crankcase. These pumps can be driven by motors, turbines, or engines.
A reciprocating piston usually has suction pressure on one side and discharge pressure on the other side. The piston must be sealed from the cylinder to prevent discharge liquid from leaking around the piston to the suction side. Rings are the most common type of piston seals. They can be made of metal, carbon, plastic or fibre.
6.3 Plunger Pumps
Plunger pumps are single and double acting. They have the highest efficiency (take less power) of any type of pump. In a single acting pump, the cylinder fills with liquid on the inboard stroke and discharges liquid on the outboard stroke. The volume of liquid that discharges will equal the volume that the plunger displaces, and is not affected by leakage around the plunger. See Figure 40 & 42.
PLUNGER PUMP PACKING
Plungers are made from a variety of materials steel, stainless steel, ceramic, plastic and alloys. Most are either high quality steel, or steel coated with hard corrosion-resistant material, or solid ceramic. Because of the wide material selection plunger type pumps are often chosen over piston types for chemicals and acids. Most plunger pumps are single acting, and may have as many as 9 cylinders, although pumps with 3 or 5 cylinders are most commonly used.
Rod seals are used to prevent liquid under pressure inside a pump from leaking out of the pump. Leakage will occur around the rod that strokes back and forth. The seal includes a stuffing box chamber surrounding the rod, which is filled with packing. Packing can be of many types depending upon pressure and type of fluid.
Packing falls into three classes. First, there is the jam or compression type, which includes any material that is inserted into the stuffing box and compressed by tightening the gland nuts until it stops leaking. Jam packing can be braided, twisted, woven, or laminated of rubber, plastics, fibre, etc.
Compression type packing is available in split rings, or in spools from which the required length is cut. The packing is usually installed in 2 sections separated by a lantern ring.
Another type of packing is the automatic type. It usually requires no gland adjustment after it is properly installed. The most common example is the V ring or chevron. It is installed on the pump rod so that pressure in the pump pushes the V rings against the rod and prevents leakage. This type of packing must be installed so that it is free to flex from pump pressure. It will leak if it is compressed too much. See Figure 42A.
A final type of seal is the floating type. It consists of several rings or segments of rings installed in cups inside a cage. Pump pressure on the outside surface of the ring pushes it against the rod and prevents leakage. See Figure 42B.
The purpose of packing is to prevent liquid inside the pump from leaking out. Packing consists of a number of rings. Each ring should reduce the pressure until it is zero at the last ring. Thus, a small leakage should occur between the inner rings. This provides cooling and lubricates the rod as it moves through the packing. If there is no liquid in the packing for lubrication, the packing and rod will get hot. This will cause packing failure and rod wear.
Rod and Packing Lubrication
It is often a good practice to adjust pump packing so that a drop or two of liquid leaks out the packing to assure lubrication. If the pump liquid will not adequately lubricate the packing, it is necessary to provide an independent lubrication system. It may be a container holding oil mounted above the rod that drips oil on the rod; or it may be a small pump that injects oil into the packing gland. See Figure 43.
ROD AND PACKING LUBRICATION
When an external pump is used to lubricate packing, oil is injected at about the midpoint of the packing. This is usually the location of a lantern ring in glands containing compression type packing. The same injection point is used to inject a buffer liquid into packing glands on pumps handling highly corrosive or toxic liquids. In this situation, leaking packing will result in the higher pressure buffer liquid flowing into the pump rather than pump leaking to the atmosphere.
On each stroke of a reciprocating pump, a suction valve or a discharge valve opens. Both valves are closed at the beginning and end of a stroke. Both valves in a pump are the same type. They are a form of check valve, that is, they allow flow in only one direction; and they open when pressure on the outlet end is lower than pressure on the inlet end.
Since each valve moves from closed to open to closed position many times each minute, they must be designed to move from one position to another almost instantaneously. The capacity of the pump will be reduced if the valves restrict the flow of liquid entering or leaving.
The most common types of valves used in reciprocating pumps are shown in Figure 44. The wing or disc type is used on most process and/or pipeline pumps. The ball-and-seat type is normally used on small chemical injection pumps.
The piston or plunger is contained in a cylinder. The cylinder is made of cast iron, cast steel, or forged steel, depending upon the pressure rating. The cylinder may be bored to fit the piston or plunger, or it may contain a liner which is replaced when it is excessively worn. It may be changed in size to change pump capacity.