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[chempro1994]

These might seem like dumb questions but they have been confusing me for some time now and any help will be greatly appreciated.

 

1. So you have this closed vessel at atmospheric pressure, or round about. The inlet to the vessel is from the discharge of a pump which pumps liquid at, say, 140 psi. The drain and vents are all closed and there isn't any other outlet. Now, you start pumping the liquid. The pressure of the liquid is 140 psi. When it enters the vessel, what happens? Will the pressure of the vessel increase? How does it increase? Does some of the liquid flash into vapor , resulting in the increase of pressure of the vessel? If so, suppose you fill the tank halfway and then stop the pump and close all inlets and outlets. What will be the pressure of the vessel now? 140 psi or will it be lower than that?

 

2. Now , suppose you don't stop the pump and let the tank fill. You don't stop even when the tank is full. You try to continue pumping. Does the pressure in the vessel increase beyond 140 psi, the design discharge pressure of the pump?  Or it cannot be increased beyond this 140 psi? Basically , is there a pressure increase due to buildup even beyond the rated discharge of the pump? If there is no pressure buildup , then what exactly happens as you continue to try to pump the liquid into the vessel? Is there backflow?

 

Thank You.

[Art Montemayor]

Chempro:

 

First, I have to ask you if you are a student.  This isn’t meant to downgrade your post, but rather to know if this topic belongs in this Industrial Professionals Forum rather than in the Student Forum.  Let’s face it: this topic is so basic and simple for a chemical engineer that it seems to belong in the Student Forum, and my intent is to keep our Forums organized.  My answers to your questions are as follows:

• You fail to state what TYPE of pump you are writing about.  We have to assume you mean a CENTRIFUGAL pump.  When ANY pump starts to pump, the pressure is NOT 140 psig.  It is the pressure initially existing in the discharge - in this case, atmospheric.  A pump doesn't create pressure.  A pump forces a flow of liquid through a conduit and it is the resulting downstream resistance to flow that produces the resulting discharge pressure on the pump (this is probably one of your basic problems regarding pumps: you don’t seem to know how they operate, and why.)

• As the pump starts to fill the vessel with liquid, the vapor space in the vessel will start to become less and its pressure will increase.  It increases because the volume of the existing vapor in the vessel (presumably air) is decreased.  Knowledge of the equation of state (EOS) for a gas is required for you to understand and know this (PV = ZnRT).

• Yes, some liquid might vaporize into the vapor state inside the vessel - but this depends on its temperature and properties (you haven’t told us the liquid’s identity).  If you stop the pump when the vessel is half full, the air(?) pressure in the vapor space can be easily calculated using the EOS.  Since we don’t do homework for others on our Forums, I’ll leave that up to you to calculate it.

• Depending on the resulting liquid properties, you will never fill the vessel 100% with liquid.  There will be some gas (air?) in it when the pump reaches its maximum head pressure (a characteristic of all centrifugal pumps - but NOT of positive displacement pumps).  The centrifugal pump will continue to turn at its maximum head pressure, but there will be no further liquid entering the vessel.  Ultimately, the pump will probably burn out or trip out electrically (if electric motor driven).

There is no way I, you, or anyone else can tell if the magical 140 psig will be surpassed in the discharge since we know nothing about the pump!

 

These questions reveal a fundamental lacking in your knowledge of fluid flow - particularly pumps and how they vary and work.  I hope you are not involved in operating any of them.  A person can create hazardous situations when trying to operate a pump without having any basic knowledge of how and why it works and I hope this doesn't happen to you.