Why discharge valve is closed before starting the centrifugal pump? | The basics of pump start up

Why discharge valve is closed before starting the centrifugal pump? | The basics of pump start up | Core Engineering
In this video we are going to discuss about the ;
Why start pump with discharge valve closed?
What happens when centrifugal pump discharge valve is closed?
What happens when you dead head a pump?
The basics of pump startup
what is the purpose of discharge valve?
Why discharge valve closed during starting the centrifugal pump?
Why centrifugal pump is started with discharge valve closed?
Why is the discharge valve kept close before operating the centrifugal pump?
Why discharge valve closed during starting of centrifugal pump?
Pump discharge valve
Why centrifugal pumps are started with the discharge valve close?
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[@loridave1962]

On all the centrifical pumps I hv used, the best practice was to open the supply fully and nearly close the discharge valve. Almost closed was best. Engage the driver and slowly open the discharge valve. Monitor the sound and pressure gauges. Open the discharge to no more than the rated flow of the pump and certainly not beyond cavitation occurring. This can take some practice and coaching. In school we were taught that a pinched back discharge is doing less work than an open discharge and that is why an electric motor driver draws less current when the discharge is closed versus open. And During electrical preventive testing I hv seen that the current is less when the discharge is pinched back. So , yes, a soft start is easier on the driver. Additionally, if the suction line is only partially full of liquid at start up, the soft start is necessary to nurse the vapor thru the pump. A liquid centrifical pump has a hard time moving vapor. A wide open discharge would merely sling out all the liquid and the pump would have no prime and not be able to develop suction. And with no suction the vapor just sits in the pump and no liquid will flow. Do not run a pump with the discharge closed, or pinched back hard, more than momentarily. Read the manufacturers instructions . Perfect your skills . Be careful about getting incorrect advice from co workers. Understand the physics of what is writen or are told before accepting it as fact.

[@hadjkouideryounes5634]

In the first time you say that the best is to start the pump with the discharge closed and another time you say don't start the pump with the discharge closed, I don't understand

[@dicko-200]

more than a moment @@hadjkouideryounes5634

[@Cosas47]

Can it help if you use a soft starter for start the pump? There are some applications that does not allow to continue operate that valve. For example, pumps that work automatically according to tanks level with no supervision

[@loridave1730]

Hadj, I think you misread. Please check. Again, it is important to start the pump with the discharge nearly closed (it is ok to start with the discharge completely closed with some pumps). Start the pump driver and promptly open the discharge valve a small amount. Monitor the sounds you are hearing and the pressure gauges (and vacuum gauge if you have one). Let the sounds and gauges stabilize. Open the discharge valve just a little more and repeat and repeat. Do not rush this process. During this start-up process the discharge pipe has a lot of vapor in it and just a little liquid. If the operator opens the discharge valve too fast, the pump will sling all of its liquid into the vapor space in the discharge pipe. When this happens the pump loses its prime and even though the pump driver and pump are rotating, the pump, with no liquid in it, can not suck more liquid into its intake. The pump impeller and volute require a little liquid to seal the internal gaps between the two. Certainly if there is a lot of relative head pressure on the supply liquid at the pump intake, the pump won't lose prime but this is not common. Hadj, I did state not to run the driver with the pump discharge closed, but it is ok to to start with the discharge closed or almost closed, just do not run the pump with it closed, but at initial momentary start-up its ok to. As I said above, it is necessary to. But, do not leave the discharge closed for more than a very brief moment. Running a pump with no liquid flowing thru it (running it dry) will cause heat to build up quickly. You may not feel the pump body getting hot (but you might), however the drive shaft, its seals, and bearings will quickly get hot if there is no liquid to cool keep everything cool. And the liquid also lubricates the shaft seal. This heat will burn (coke the residual liquid) the shaft seals which then results in a leak which then results in an inoperative pump and no productivity which results in repair costs and.may result in a new operator. Running a pump dry, because liquid supply ran out or because a supply valve was shut, dictates that the pump driver be shut off immediately. Shutting supply or running out of liquid are what I have seen cause the most --if not all--pump leaks. In these cases I have later always found the pump shaft seal(s) with coked liquid and deformed polymers. . Inattentiveness or lackadaisical operator error every time. A good pump operator makes for a happy boss.

[@pratikmetangale]

There is NRV present in discharge line already, so is it make point that valve is close or open... Coz nrv will do its work.

[@thomassim5600]

Thanks for the knowledge 😊

[@punjab4816]

thank you so much sir 🙏

[@prasadshetty8823]

Thank you for the knowledge sharing....mean while I have a one doubt , is there any effect on impeller or mechanical seal if we close the discharge valve?

[@affanbhaijaan65]

Good🎉

[@irfanbashir9288]

you explained really well. how reverse flow can damage the pump which specific part can be damaged

[@sagardipjana2059]

But sir, we are using NRV Then how is it possible??

[@user-dv7bt6pc6m]

How liquid can back flow from discharge to the suction at the same time high current value of motor ,, i mean the motor at this time working at highest capacity?

[@baijuupadhyay6891]

Sir plz make video on self priming pump, humble request 🙏

[@Kasiramaniyer]

Not to suggested close the discharge line.. Depend on valve we used at discharge end..

[@maqboolvahora9925]

After starting pump, Recycle flow control valve piping hammering and High vibration to maintain minimum flow. Kindly explain and give advice , solution

[@Coreengineers]

Check the pump curve for that pump and also check the sizing of line..... hammering have many reasons improper sizing, high velocity, no support could be few...

[@user-pn6sv5oc9z]

how many time that's we required to close the valve?

[@Coreengineers]

When?

[@badawg4876]

One point you made during the video is misleading. You stated that the pump is at minimum resistance when the discharge valve is closed. This is incorrect. The pump has minimum resistance when the discharge valve is OPEN. When you close the discharge valve, the pump will have MAXIMUM resistance. Essentially you are deadheading the pump (max resistance, no flow). In other words, closing the valve provide resistance. YOU ARE RESTRICTING THE FLOW.

[@shaneabbas3985]

Make it in hindi...That would be great and be clear on the point..

[@Coreengineers]

Please visit @Learn EEE for Hindi video

[@youssef7253]

Noo lot of us dont understand hindi Thank you for uploading english vids

[@zeeshanmalik5426]

Suppose i close the discharge valve for longer period of time and meanwhile the pump is running what harm it can cause to the pump ?????

[@rashadalakbarov9394]

The fluid will start to heat up , and that could lead to damage to seals , internals etc

[@ravindrajoshi822]

How to drain discharge line???

[@Coreengineers]

There are multiple ways to do that....some times in discharge line that provision is provided or pump manufacturers also give that insights.

[@ravindrajoshi822]

What I really asked where to drain provision provided before or after NRV??

[@renjithsuvarnan7119]

How there will be back flow when pump discharge has NRV😅

[@Coreengineers]

Due to leakage or may be due to back pressure

[@keshavgokhaledivine]

Why are you asking the questions? Who's going to reply and how?

[@Coreengineers]

So what's you want the spoon feeding? Those who'll be intrested to watch and learn.....they should give the answer.

[@subratanaskar-hm8tm]

If discharge valve is open then suddenly flow will reach high value and we know power consumption directly proportional to the flow rate .and due to this if pump hydraulic power exceeds the motor power then pump will be tripped To avoid tripping issue we need to throttle the discharge valve before starting the pump.

[@bradshsi]

The video is a gross oversimplification. High NS pumps will have a higher power consumption at shut valve. Other than that the video contains numerous errors such as stating: 5:18 "no discharge head or minimum discharge head" Incorrect. The pump produces its maximum head at shut valve. 6:20 "closing the discharge valve avoids additional load on the pump" Incorrect. Radial and axial loads on the pump are highest at shut valve 6:40 The scenario of reverse flow can't happen since you have a non return valve.