For transferring liquid from one place to another three methods are used. Pressure, Vacuum, Pump. Pumping is very common & popular in chemical process industries. The various types of pumps are as shown below:
Types of pump
Centrifugal pump Positive displacement pump
Coupled Mono block Magnetically Reciprocating Rotary Pump Pump coupled pump Pump pump Air operated Plunger Diaphragm Gear Shuttle block Lube Screw Double pump type metering pump pump pump pump diaphragm pump
Single diaphragm Double diaphragm pump pump
Centrifugal pump
The centrifugal pump develops its pressure by centrifugal force on the liquid passing through the pump and generally applicable to high capacity, low to medium head installations. For high discharge head centrifugal pumps can be multistage instead of single stage. e.g multistage centrifugal pump is used for boiler feed water.
Basic parts of a centrifugal pump
Arrangement of pump
Hydraulic characteristics
Pump affinity laws
Selection of pump
Basic parts of a centrifugal pump
Impeller
The three common types of impellers used are:
Fully closed: Used for high head & high pressure applications.
Semiopen: Used for general purpose applications. It has got open vane tips at the Entrance to break up suspended particles & prevent clogging.
Open: Used for low heads, suspended solid s applications, very small flows. These impellers are available in any nearly any material of construction as well as rubber, rubber lined, teflon lined, glasslined. The lined impellers are of open type.
Casing
The casing may be constructed of a wide variety of metal, lined material, plastic materials etc.
Shaft
Care should be taken in selecting the shaft material. It must be resistant to the corrosive action of the process fluid yet posses good strength characteristic for design. Generally sleeves are used over shaft to prevent direct corrosion / mechanical danger to shaft. These sleeves may be metal, ceramic, rubber etc.
Bearings
The bearings must be adequate to handle the shaft load without excessive wear provided lubrication is maintained. In all cases bearings should be of the outboard type, that is not in the process fluid unless special conditions prevail to make this situation acceptable.
Packing & seals on rotating shaft
Conventional soft or metallic packing in a stuffing box is satisfactory for many low pressure, non-corrosive fluid systems. Special packings such as mechanical seals are commonly used for corrosive fluids when the pressure becomes high above (about 50 psig) or the fluid is corrosive, additional means of sealing the shaft must be provided. Particular care must be taken in handling and using the mechanical seals.
Arrangement of pump
Pumps in series
Sometimes it is advantageous or economical to use two or more pumps in series to reach the desired discharge pressure. In this situation the capacity is limited by the smaller capacity of any one of the pumps at its speed of operation. The total discharge pressure of the last pump is the sum of individual discharge pressure of the individual pumps. The pump casing of each stage (particularly the last) must be of sufficient pressure to withstand developed pressure.
Pumps in parallel
Pumps are operated in parallel to divide the total load in two or more smaller pumps. The individual pump operates on its own characteristic curve. The capacity gets added up.
Hydraulic characteristics :
Capacity
Total head
Suction head or suction lift
Discharge head
NPSH & pump suction 1. Capacity
It is the rate of liquid or slurry flow through a pump. For proper selection and corresponding operation a pump capacity must be identified with the actual pumping temperatures of the liquid in order to determine the proper power requirements as well as the effects of viscosity.
Pumps are normally selected to operate in the region of high efficiency and particular attention should be given to avoiding the extreme rights side of the characteristic curve where capacity & head may change abruptly.
2. Total head
The pressure available at the discharge of a pump as a result of change of mechanical input energy into kinetic & potential energy. This represents the total energy given to the liquid by the pump. Head is expressed as feet of liquid being pumped. The head is independent of the fluid being pumped & is therefore the same for any fluid thorough the pump at a given speed of rotation & capacity.
3. Suction head or suction lift
The total suction head is the difference in elevation between the liquid on the pump suction side and the centerline of the pump + the velocity head. When the liquid level is below the pump centerline the difference in elevation is known as suction lift.
Total suction head (TSH) = Static head – friction head loss.
Total suction lift (TSL) = Static lift + friction head loss.
4. Discharge head
The discharge head of a pump is the head measured at the discharge nozzle 7 is composed of static head, friction losses through pipes, fittings, contractions, expansions, entrances &
exit, thermal system pressure.
5. Velocity head
As a component of both suction and discharge heads, velocity head is determined at the pump suction & discharge flanges respectively and added to the gage reading. The actual pressure head at any point is the sum of the gage reading + the velocity head. The values of velocity heads are usually small and negligible often.
6. NPSH and pump suction
The net positive suction head (NPSH) is very important criteria for centrifugal pumps. There are two types of NPSH. NPSHa and NPSHr.
NPSHa = Net positive suction head available = Pressure on suction side – friction loss – vapour pressure of liquid being pumped at pumping temp.
NPSHr = Net positive suction head required. It is specified by the pump manufacturers. The NPSHr is very important consideration in selecting a pump, which might handle liquids at or near boiling points or liquids of high vapour pressures. If this consideration of NPSHr is ignored, the pump may well be inoperative in the system or it may be on the borderline and become troublesome and expensive. The significance of NHSHR is to ensure sufficient head of liquid at the entrance of the pump impeller to overcome the initial flow losses of the pump.
This allows the pump impeller to operate with a full bite of liquid essentially free of flashing bubbles of vapour due to boiling action of the fluid poor suction condition to cavitation in
pump impellers and this is a contributor at which pump can not operate for a very long without physical erosion, damage to the impeller.
Cavitation in a centrifugal pump or any pump develops when there is insufficient head for the liquid to flow into the inlet of the pump allowing flashing or bubble formation in the suction system and entrance to the pump. Each pump design or “family” of dimensional features related to the inlet and impeller entrance pattern requires a specific minimum value of NPSH to operate satisfactorily without flashing, cavitating and loss of suction flow.
Under cavitating conditions a pump will perform below its head performance curve at any particular flow rate. Although the pump may operate under cavitating conditions, it will often be noisy because of collapsing vapour bubbles and severe pitting and erosion of the impeller often results. This damage can become so severe as to completely destroy the impeller and create excessive clearance in the casing. To avoid these problems, the following are few situations to watch.
Have NPSH available at least 2 feet of liquid greater than the pump manufacturer
Requires under worst possible operating conditions.
Internal clearance wears inside pump.
Entrained gas (non condensables)
Deviations or fluctuations in suction side pressure, temperature increases, low liquid level.
Piping layout on suction, low particularly tee intersections, globe valves, baffles etc.
Liquid vortexing in suction vessel, thus creating gas entrainment into suction piping.
Nozzle size on liquid containing vessel may create severe problems if inadequate. Liquid suction velocities in general are held at 3-6.5 ft/sec. Usually as a guide the suction line is at least one pipe size larger than the pump suction nozzle.
The NPSH required by the pump is a function of the physical dimensions of casing, speed and type of impeller and must be satisfied for proper pump performance. The required NPSH of a pump increases as the pump speed increases.
RPM NPSHr0.75
For this reason many critical suction condition installations use relatively slow speed pumps.