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If you don\'t know how to answer it please don\'t bother to answer to attempt.

OVERRIDE CONTROL

Problem :

The pipeline shown below has three booster pumps installed at some distance from each other. Each pump runs at a fixed speed and the exit valve, which is fail-closed, is manipulated by the operating personnel. However, there are two conditions that must be avoided for the safe operation of the pumps. The first condition is preventing the suction pressure for being too low – below Pmin – because the pump could cavitate. The second condition is preventing the discharge pressure from being too high – above Phigh – because the pipe could burst. If any of these conditions is approached, then manipulation of the exit valve is required to avoid the condition. Design a control system to avoid these conditions.

Solution

Open impellers

Vanes are attached to the central hub, without any form, sidewall, or shroud, and are mounted directly onto a shaft. Open impellers are structurally weak and require higher NPSHR values. They are typically used in small-diameter, inexpensive pumps and pumps handling suspended solids. They are more sensitive to wear than closed impellers, thus their efficiency deteriorates rapidly in erosive service.

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Partially open or semiclosed impellers

This type of impeller incorporates a back wall (shroud) that serves to stiffen the vanes and adds mechanical strength. They are used in medium-diameter pumps and with liquids containing small amounts of suspended solids. They offer higher efficiencies and lower NPSHR than open impellers. It is important that a small clearance or gap exists between the impeller vanes and the housing. If the clearance is too large, slippage and recirculation will occur, which in turn results in reduced efficiency and positive heat buildup.

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Closed impellers

The closed impeller has both a back and front wall for maximum strength. They are used in large pumps with high efficiencies and low NPSHR. They can operate in suspended-solids service without clogging but will exhibit high wear rates. The closed-impeller type is the most widely used type of impeller for centrifugal pumps handling clear liquids. They rely on close-clearance wear rings on the impeller and on the pump housing. The wear rings separate the inlet pressure from the pressure within the pump, reduce axial loads, and help maintain pump efficiency.

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Number of impellers

Single stage pumps

The single-stage centrifugal pump, consisting of one impeller, is the most widely used in production operations. They are used in pumping services of low-to-moderate TDHs. The TDH (total dynamic head) is a function of the impeller’s top speed, normally not higher than 700 ft/min. Single-stage pumps can be either single or double suction. The single-stage pump design is widely accepted and has proved to be highly reliable. However, they have higher unbalanced thrust and radial forces at off-design flow rates than multistage designs and have limited TDH c