3 replies to this topic

[chem2013]

Hi Everyone.

 

A level control valve is placed at inlet of a low pressure storage tank (N2 blanketed to positive pressure). Inlet liquid is coming from a different unit within plant from a pump discharge. This pump is supplying sub cooled liquid to various users.

 

This tank is provided with a PCV located at vent line.

 

Pressure at inlet to level control valve is 3.2 barg. If level control valve fails open (control valve failure) high pressure liquid around 3.0 barg would hit bottom of low pressure storage tank.

 

My understanding is in this case, tank PCV would not protect the tank. Tank may fail due to high inlet liquid pressure as hydrotest pressure would be exceeded. So, inlet pressure should be lowered at inlet of level control valve or the supply should be from a dedicated pump (pump should be sole supplier).

 

Please advice your opinion.

 

Regards.

 

 

[fallah]

chem2013,

 

After LCV fail open the flow will be increased such that the corresponding pressure at the inlet point to the tank will be the tank operating pressure plus the pressure of static head above the inlet connection to the tank bottom. If the PCV at the tank vent line had already been sized for such excessive incoming flow there would be no problem otherwise the tank may be pressurized over its design pressure. Of course, if the tank had been equipped with an proper size emergency vent , it can handle the case and tank will be protected against that higher incoming rate.

[Supriyo Mukherjee]

I agree with Mr. Fallah here. 

Normally when we size PCV for these kind of tanks we follow the procedure as provided in API 2000. It lays down all the possible ways of liquid entry into the tank and also vapor generation. Also if any other sources of inlet fluid is there that is also considered in those cases.

Also as th PSV (which presumably is there in the tank) will take care of the over pressurisation issue in case PCV cant control .

[chem2013]

_{Thanks for clarification.}