1. Capacity = 38.4 m3/hr 2. Head = 16.65 m 3. Efficiency = 38.76 % 4. BHP = 11 HP
Hence recommended motor = 15 HP Or 10 HP with relay.
For above mentioned viscous liquid in table the horsepower required is 170 % more than that required for pumping equivalent water. The viscous efficiency is 44 % lower than equivalent water efficiency.
Effect of temp. rise & minimum safe flow
When a pump operates near shut-off capacity and head or is handling a hot material at suction, it may become overheated and create serious suction as well as mechanical problems. To avoid overheating due to low flow, a minimum flow should be recognized as necessary for proper heat dissipation. However it is not necessarily impossible to operate at near shut-off conditions provided
It does not operate long under these conditions as temp. Rise per minute vary from less than 1°F to 30-40°F.
A bypass is routed or recycled from the discharge through a cooling arrangement and back to suction to artificially keep a minimum safe flow through the pump while actually withdrawing a quantity below the minimum.
MOC
The pumps come in metallic & non metallic material of constructions. The MOCs are:
Metallic: CI, CS, SS-304, SS-316, Hast B, Hast C, Zirconium, Alloy 20 Non metallic: PP, PVDF, PTFE, CI/PP, CI/PVDF, CI/PFA, GRP, Graphite,
The efficiency of metallic pumps is higher than non metallic pump because in metallic pumps closed as well as semi-open impellers are possible. In non metallic pumps only open impellers are possible because of basic problem of moulding. The efficiency of open impellers are lower than closed / semi-open. The metallic pumps are more sturdy so mechanical damage chances are low. Many times the pump manufactures do not take the responsibility of MOC. In such cases the MOCs are to be mentioned by the customer himself.
Operating condition of fluid / Nature
The operating condition details such as temp, pressure, density, viscosity, % slurry if it is applicable, nature of solids whether abrasive or soft must be clearly mentioned in the data sheet.
The corrosive nature of the pumping fluid also must be clearly mentioned. Incomplete information may lead to wrong selection of pump which may ultimately fail at plant level.
System condition
The pump manufacturer must know if the suction side of the pump is associated with vacuum equipment or is to lift the liquid. This can make a difference as to the type of impeller, Suction opening to be provided. If the system operates intermittently it should be noted. A piping diagram is often helpful in obtaining full benefit of the manufacturer’s special knowledge.
Pumps operating under suction lift
As centrifugal pumps need priming, there are various methods to prime. Foot valve can be used so that the liquid does not drain out after pump stoppage. Auto-priming pot can be used. The auto priming pot is installed above the pump centerline. One line from top of pot goes to the suction tank; another line from bottom of priming pot goes to the pump suction. Initially some liquid is filled in priming pot, pump is started. The liquid from pot gets sucked into pump, vacuum gets created in the pot. This vacuum causes the liquid to get lifted from suction tank into auto-priming pot. This common method of auto-priming is used for pumping water from underground tanks.
Motor details
The details of the electric motors should be clearly mentioned in the data sheet if the scope of supply of motor is in client’s scope. The frame size of motors should be also very clearly mentioned. If the frame size is not mentioned in the data sheet ensure that the pump manufacturer knows about it. Also ensure that latest frame details are available for motor. Some times the motor manufacturer changes the frame details and the latest information may not be available with client as well vendor. In that case wrong frame size will be supplied & when the pump is delivered the motor may not fit exactly & modifications may need to be carried out at site.
The RPM of motor must also be mentioned clearly in the data sheet whether 1440 or 2900 RPM.
Positive displacement pump
Under this category two types of pump come: Rotary pumps
Reciprocating pumps.
Rotary pumps
There are many types of rotary pumps e.g. Cam, Screw, vane, Lobe, Shuttleblock etc. The majority of this type is capable of handling a clean solution essentially free of solids. These pumps handle materials of a wide range of viscosity ( upto 500.000 ssu ) and can develop quite high pressures ( over 1000 psi ). In addition the unit s can handle some vapour or dissolved gases with the liquid being pumped. The capacity is generally low per unit and at times are used for metering.
These pumps are low in cost, require small space and are self-priming.
Some can be operated in either direction, have close clearances, require over-pressure relief protection on discharge and have low volumetric efficiency.
The performance characteristics are:
Performance characteristics
Flow is proportional to speed and almost independent of pressure differential.
Internal slip reduces efficiency.
Entrained gases reduce liquid capacity and cause pulsations.
BHP varies directly with pressure and speed.
For speed & pressure constant, BHP varies directly with viscosity.
Selection
Suction and discharge heads are determined the same way as that for centrifugal pumps. Total head and capacity are used in selecting the proper rotary pump from a manufacturer’s data or characteristic curves since viscosity is quite important in the selection of these pumps. It is sometimes better to select a large pump running at low speeds than a smaller pump at high speed when dealing with viscous materials. As a general guide, speed is reduced 25-30 percent below rating for each ten fold increase in viscosity above 100 ssu. Mechanical efficiency of pump is decreased 10 percent for each ten fold increase in viscosity above 1000 ssu.
Reciprocating pumps
In this category the pumps are: Plunger pump
Diaphragm type metering pump Plunger pump
These can be simplex, duplex or multiplex types. These are gland pack pumps. The liquid being pumped is coming in contact with the plunger. There is always some leakage through gland.
These pumps cannot be used for dangerous, poisonous liquids. These are used generally for slow feeding of chemicals.
Diaphragm type metering pump
This type of pump is the most commonly used pump in the category of reciprocating pumps. The advantage is that zero leakage is possible with these pumps. The pumping liquid is separated from the plunger via teflon diaphragm. The plunger is operating in hydraulic oil chamber. On the suction stroke plunger moves backward, thereby creating vacuum in hydraulic chamber. This pulls the diaphragm back, thereby sucking the liquid. In forward stroke the piston moves forward, thereby pushing the diaphragm in the forward position. This causes pressure to develop on discharge side, thereby opening discharge non return valve and forcing liquid out. The action is repeated. Since there is no leakage these pumps can be used for pumping toxic chemicals as well. Diaphragm type metering pumps can be simplex, duplex, triplex, multiplex etc. The liquid coming in contact with the plunger is hydraulic oil ad not the process fluid. There is some leakage of hydraulic oil to atmosphere, which is acceptable.
For handling extremely corrosive chemicals double diaphragm type teflon coated metering pumps are used. As the name suggests there are two diaphragms. One diaphragm separates the hydraulic oil from intermediate fluid. Another diaphragm separates intermittent fluid from process fluid. The fluid used in the intermediate chamber should be compatible with the pumping fluid.
These pumps come in metallic as well as non metallic MOC used for very corrosive liquids. The metallic MOCs are CS, SS-304, SS-316, HastC etc. The non metallic MOCs are PP, PTFE, CI/PP, CI/PTFE etc.