Important Factors When Selecting Chemical Pumps

Following are the important factors taken in to consideration while selection chemical pumps :

1. Capacity and discharge head

This expected in usual terms

Suction process pump may be required to lift the liquid from a lower level or liquid coming under pressure at the suction end. It is, therefore, necessary to know the working pressure and the operating head. For example, if the working pressure of the pump is 25 Kg/cm2 and the inlet pressure is 10 Kg/cm2 the operating head can be only upto 15 Kg/cm2.

One of the most important parameters to be taken into consideration is the Net Positive Head. Generally speaking NPSH is defined as pressure at the suction inlet of the pump, less vapour pressure. It is known that atmospheric pressure can support water column to a height of 34 ft. In practice, this is not true for several reasons. The suction pressure should not go below vapour pressure of the liquid. Otherwise the liquid will get converted into vapuor form and the flow will be discontinued. The pump is not designed to handle vapour. There is some loss of pressure due to friction in the pipe line, foot valve, bend etc. Some head gets converted into velocity head. As a result the head available may be given as 34-ft. vapour pressure –velocity head –friction head +inlet pressure. This can be described as Net Positive Suction Head available. Every pump by its design characteristic requires certain NPSH at the inlet for pumping liquid. This is called, as NPSH available must be more than NPSH required. While selecting chemical pump therefore, both the values must be known. Generally NPSH required by Chemical Process pump is 2.5 metres. However, it differs with different models of pumps and also it differs with speed.

2. Chemical composition of the liquid 

In order to ascertain the reaction of the liquid on the wetted parts of the pump, it is necessary to know the chemical composition of the liquid, it is also desirable to know the pH valve which indicates the acidic or basic nature of the liquid. Pure water has a PH valve of 7. Between 0 to 7, the liquid is acidic and between 7 to 14 the liquid is basic. 

3. Viscosity 

SSU. It can be used viscous liquid from 500 SSU to1500 SSU with due correction. However, in this range the pump has to be heavily departed. It results in selecting a big pump for relatively smaller capacity and head. It is not advisable to use centrifugal High viscosity of liquid can cause high friction loss. As a result the pump develops less head and less capacity. The power consumption increases.In other words the pump required to be derated. Chemical process pump can safely handle viscosity up to 500 pump for viscous higher than 500 SSU. 

4. Temperature 

Temperature affects the pump in following way:

  • In case of certain chemicals corrosion action depends upon the temperature. Usually hot boiling liquid has more corrosive action. 
  • If the temperature raises, heat is transferred from gland packing through the shaft to the bearing.Such a transfer of heat to the bearing reduces bearing life considerably. It may even seize the bearing.In such cases, it is necessary to make sufficient cooling arrangement at the gland, so that heat is carried away. It may be down either by use of direct injection or cooling water in to gland or by providing cooling jacket for the cold water circulation. 
  • Temperature affects viscosity. Many times the viscosity is given at certain laboratory temperature.However, If the working temperature is quite different, the viscosity figure given is of no use. In such a case, the knowledge of actual temperature will be quite useful in getting the correct figure for theviscosity. 
  • At high temperature, the expansion of metal parts takes part in different proportion. This factor must be taken in to consideration when offered a pump for high temperature 
  • Bedside temperature, it is necessary to know whether it is required to preserve heat (or to maintain the low temperature as the case may be). Such a condition may be necessary to maintain the flow condition or to achieve operation economy. In such cases, steam jacketing may have to be used to maintain high temperature of the while passing through the pump or proper insulation to be provided to maintain the low temperature.

Chemical process pump can operate in the range of minus 50 deg c to plus 300 deg c. if the temperature is above170°C, it is necessary to provide cooling arrangement at the gland. A stuffing box jacket is provided through which cooling liquid can be passed. Also in such cases, centerline mounting foot support is to be provided. Volute casing is mounted on external feet (not integral with casing) and the support is at the level of shaft centerline. Cold water is circulated through the external feet. This design takes into account uneven expansion of mounted parts. 

5. Vapour Pressure 

As soon as the earlier vapour pressure of the liquid influence the working of the pump on the suction side, it is the pressure at which a given liquid vapourises. The vapour pressure again depends upon the temperatures. Higher the vapour pressure, lower the NPSH available. If pressure is more than the available NPSH, cavitation occurs. In case of high vapour pressure, the liquid is supplied to the pump with some suction head created by keeping the pump at a lower level thereby creative positive head at the suction head. The other solution is to reduce the temperature of the liquid before it enters thepump.

6. Specific gravity 

Specific gravity directly influences the power consumption. Hence the liquid with higher specific gravity requires proportionately high H.P. It should be noted that there is no change in the capacity delivered and the head developed, if the specific gravity is higher or lower. Specific gravity also affects the NPSH available. All the while we have taken the figure of 34 ft. of water column while calculating NPSH available. However, if the liquid is heavier than water, the atmospheric pressure again support that liquid to a higher of 34 ft. as is amply clear from the fact that with a heavier specific gravity like mercury in the liquid form, the height is only 32”. If therefore one is required to calculate NPSH available, specific gravity has to be taken into consideration.

7. Solid contents 

The solid contents of the liquid influences the type of impeller and gland arrangement. The solid may be in two forms. One in the form of fully dissolved state. In this case, it will only affect the specific gravity and the viscosity of the liquid. If the solid remains in undissolved form, it is necessary to know the type of solid, such as hard, soft, abrasive or nonabrasive. A centrifugal pump can normally handle between 10 to 15% by volume. 

Hard and abrasive solids wear out the pump parts rapidly. To protect the costlier parts, the manufacture may have to provide suitable renewable liners. The manufacture will also have to take care to prevent the entry of solid in the gland region, so that the life of the gland ring and the shaft protecting sleeve is not shortened. In such cases, double seals may be used. It will also be possible to provide circulation through the seal to carry away abrasive particles. Generally when there are solids in suspension, semi-open impeller, is offered. 

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Post time: Aug-06-2019

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