MechChem Africa June 2019

⎪ Pump systems, pipes, valves and seals ⎪

Harry Rosen of TasOnline compares the management methods for modern pumping systems to the election of an autocratic residents association: both can find themselves out of touch with real operational needs. Modern pumping systems and residents associations

B eing chairmanof our local residents association has been a great op- portunity to study various people’s personalities.Ihavealwaysbelieved that the role of the chairman is to support the members of the committee in carrying out their tasks to the best of their ability. All members are selected for their abili- ties, which should enable them to function without the need for supervision or being micro managed. In our residents association, this worked successfully for the first eight months. As chairman I was quite proud of the fact that I did very little ‘real work’, believing I was only there to ensure others could get their jobs done. Then a new committee came in with a top-heavy autocratic leadership style, which has taken responsibility away from individual members and made people reluctant to stick their headout. Toput itmildly, the association is in crisis. I recently presented a pump training course to pump operators at a bulk water supply company. As with many engineering companies in South Africa, there have been many changes over the past few years, in this case specifically around the job description and responsibility of pump operators. In the past, pump operators would be on site, manually starting up and shutting down pumps,whilecheckingontheircondition.Now pumps are started remotely, so pump opera- torsspendmostoftheirtimeinacontrolroom. They are no longer able to see whether there is excessive leaking from a gland assembly, or the suctionvalve isnot 100%open, or tocheck that the discharge valve is closed on startup and gradually opened as downstream pres- sure increases. This is all assumed to happen automatically. But what if the suction valve is not 100%open, or a strainer is blocked, or one of a host of other problems has arisen?Who is there to pick up on the problem? Changes to job descriptions and increases in responsibility have a dramatic effect on the operation of a pump station. Very important is training, to ensure everyone knows why these procedures are important, and what the effect on the pump operation will be if they are not followed. Here is a typical list of things the operator still needs to do or check.

Pump operators at a training course checking out a pump operation at a bulk water supply station.

How is the suction pressure when the pump is off? This gives the static headon the suction side and should be compared with the height of liquid in the suction vessel to ensure the suction gauge is working. If the gauge reads 35 kPa (3.5 m) and the height of liquid is 5.0 m, for example, then the gauge is wrong and should be replaced. Similarly, if the gauge reads 35 kPa (3.5m) and the height of liquid is only 2.0 m, then the gauge is also faulty. Check the suction pressure when pump startsup: Thesuctionpressurereadingshould drop due to friction losses in the suction pipe once the fluid starts moving. If it does not drop at all thenmost likely the gauge is faulty. • If it drops by 5.0 to 10 kPa it means the suction valve is 100% open, and suction conditions are acceptable. • If it drops more than 20 kPa, it is indicat- ing a problem on the suction side such as a blocked strainer, closed suction valve, a blocked tank outlet, etc. In most cases excessive losses on the suction side will lead to insufficient NPSH available, which could lead to cavitation. Problems such as these need to be fixed. Strictly follow the shutdown procedure: If possible, always remember to close the dis- charge valve by 80% before turning off the pump.Manysystemsincludeautomaticvalves designed to achieve this without operator in- put. This dramatically reduceswater hammer and mechanical stress on the pump. Anyone who has stood in a pump chamber and heard the loud bang as the non-return valve slams shutwill attest to the large forcebeingapplied to the piping systemwhen a pump is stopped

with the discharge valve open. Understand the cause of unplanned shut- downs: If the amps dropbelowa certain value the pump will shut down. Low current is an indication that the pump is running at low speed on the far left of its curve. This could lead to short bearing and seal life, as well as the pump overheating due to operating close to shut off. The minimum allowable amps should be calculated slightly higher than the shut off power value. If the amps exceed a maximum preset value,thepumpwillalsoshutdown.Thiscould be the maximum rated amps for the motor. Running the motor at or above rated amps will severely shorten the lifeof themotor. This could be caused by: • The amps (power) drawn by the pump when operating at high speed on the far right of the pump curve. In this case, shut- ting down will protect the pump from cavitating. • Highampssettingscouldbebasedonsome arbitrary value that is no longer relevant. In this case, the high amps preset may not be projecting the pump or motor from anything. Check where this preset value came from. As with any activity where people are in- volved, they need to be given the opportunity to do their jobs to the best of their ability. In thecaseofpumpoperators,theyneedtraining and accurate, calibrated instrumentation to ensure theynot only start and stop thepumps in themost efficient and reliableway, but also that the pumps operateoptimally throughout their lifetimes. q

June 2019 • MechChem Africa ¦ 15