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problem 1: A pumping system consists of two (2) identical pumps installed in parallel configuration.

Normally the pumps operate in duty/standby configuration so only one (1) pump is in operation.

It is decided to change the operation of the pumps to duty, duty so two (2) pumps operate at the same time. No changes where made to the distribution pipework.

a) What expected changes in the discharge pipe work are expected ?
b) Does NPSH required by the pumps increase or decrease changing from single pump to two (2) pump operation ?
c) What recommended changes need to be made to the system for the two (2) pump operation  ?

problem 2: We have a small centrifugal pump that has worked well enough (not great, but good enough) for a few years. It has negative suction (fluid is below the pump). Suction pressure is atmospheric pressure, and the pump discharges into an atmospheric tank.  It has flushed packing (not mechanical seals). 

Our site begins a water conservation program to reduce water usage. Pump flushing water is one target. We change the packing to a packing that does not require flushing. We then shut off the flushing water.

Within hours, the pump is cavitating. The level in the supply sump is well within normal levels, and flowratres are normal.  Power consumption is up but that could be due to the cavitation.  All valves on the suction and discharge are checked and found to be unmoved from the last position. We check the sump contents and it is normal – nothing that would suggest caviation.

Our first reaction is to wait to see if it goes away, but the cavitation continues for days.

What happned?

problem 3: An internal combustion engine often has a water pump and lube oil pump driven from the engine crankshaft.  Therefore, the speed of the water pump and the speed of the oil pump are linked to the speed of the shaft. If you have even been in an automobile, you know that this makes the speed of both pumps (not totally but very) random.

In your own words, describe why the water pump is often a centrifugal pump and the oil pump is often a positive displacement pump.

problem 4: We have a pumping system that handles dirty fluids with sticky soft particles. For this reason, we are using semi-open impellors on our centrifugal pumps. Our company performs a project that increases the cleanliness of the fluids, and greatly reduces the quantity of sticky soft particles in the liquid that is to be pumped (a filtration project upstream of the pumps). 

Now that the fluid has changed, list 3 potential modifications to the pump that would improve pump efficiency/ performance.

If you could only do one, which one would you do and why?

problem 6:

Part (a): Describe, in your own words, some of the issues for installing a pump in anaircraft that you would not encounter when installing a pump on land, and how you would manage those differences. Full credit will require a minimum of three differences.

Part (b): Describe, in your own words, some of the issues for installing a pump on a truck (or trailer) that you would not encounter when installing a pump on land, and how you would manage those differences. Full credit will require a minimum of three differences.

problem 7: As a legal requirement fire water pumping systems are to be checked every 12 months. The checking of the fire water pumps involves ensuring that flow (10 L/s per hydrant) and pressure (700 kPa.g at the most disadvantaged hydrant) are maintained.

What issues within the fire water system could be a potential for the fire water pumps to fail the annual pressure and flow testing ?

Note, fire water pumps only operate periodically.

problem 8: One of the key safety issues with pumps is the maximum pressure that can be created by the pump. 

Excessive pressure can exceed the mechanical integrity of the rest of the system, causing a spill (and the subsequent consequences of a spill). Some methods of limiting maximum pressure work in all circumstances, while others work in only some circumstances. 

This is a list of proposed methods to limit maximum discharge pressure in all circumstances. Unless noted,

1) It is assumed the density of the pumped liquid is constant. 
2) It can also be assumed that the fluid is such that it will not cause an obstruction in the piping system at all.
3) Finally, it can be assumed that unless stated, the pump runs at constant speed.

Your task is to determine which methods will work in all circumstances, and which will not. If it will not, you will need to describe the circumstance where the safeguard will not work.

(NOTE: This problem requires you to understand the actual methods of how the pump works).

a) Shaving the impeller of a centrifugal pump to limit pressure rise.

b) Installing a pressure relief valve on the inlet (not the outlet) of a centrifugal pump.

c) Installing a pressure relief valve on the inlet (not the outlet) of a reciprocating positive displacement pump.

d) Installing a recycle line from pump discharge to pump suction.

e) Installing a pressure relief valve on the compress air supply of an air-operated-double-diaphragm pump.

f) Installing a pressure relief valve on the outlet of a reciprocating positive displacement pump, with the discharge sent to the inlet of a pump.

g) Installing a pressure switch high on the outlet of the pump which, when activated, will shut of the power to the pump motor.

h) Installing a pipe on the outlet of a centrifugal pump. The pipe points upward, is 20 metres high, is large in diameter, and is open ended.  Any discharge will overflow into a containment pit.

Mechanical Engineering, Engineering

  • Category:- Mechanical Engineering
  • Reference No.:- M9691

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