Tank inlet nozzle location

[Hurricanes]

Is there any engineering/hydraulic reason for putting the inlet to a tank at its base?

A friend works at a processing plant, and the consulting engineers designing a new processing circuit have put the inlet to a tank at its base. Liquid is pumped into the tank, theres a check valve to prevent backflow. But the liquid tends to scale up over time.

My friend asked me if there could be any hydraulic reason for doing this, instead of having the inlet at the top of the tank. I couldn't think of any reason for doing it this way. Theres no space/geometry issues.

Does anyone know if there could be a valid engineering reason for having the inlet at the base of the tank?

 

[bimr]

It is probably a bad practice to install a check valve at the tank inlet connection. Unless the tank is tall, there will not be enough backpressure to close the check valve. If there are solids present, the solids may foul the check valve. Check valves are usually installed at pumps.

Tank connections are usually located so that the flow into the tank will not short circuit through the tank into the outlet connection. In some cases, if you may not want the tank contents to reverse flow, so you would install the tank inlet connection on the top. Top inlet connections are probably more common, but there is no hard rule that the inlet has to be on the top.

If you want to control the water level in a tank, it is customary to have the outlet connection on the top near the proposed level. Otherwise, the outlet connection may be located near the bottom.

Sometimes, when you are repurposing an existing tank, it is not cost effective to install a new connection and you end up using whatever existing connection is on the tank.

 

[JStephen]

It is commonly done both ways.
With an inlet at the top, you're always pumping against a head equal to the tank height, even if the tank is nearly empty. You have some extra pipe to install and maintain and possibly insulate.
With an inlet at the top, you may have a lot of splashing, generate a lot of vapor, etc- no problem in a water tank, undesirable in a petroleum tank. In those cases, where there is a top inlet, there will be a drop pipe inside as well.
Tanks with floating roofs obviously need the inlet below that point instead of above.
Where potable water from an outside supplier changes custody, it is common to use a top inlet with air gap as a more positive backflow prevention measure.

 

[MJCronin]

I agree with JStephen ....

But I would like to add: Your two alternatives are top inlet or bottom inlet.

Generally speaking, a bottom inlet to a tank is cheaper, both in capital costs and in long-term pump operating costs.

Top inlets also commonly require a "dip pipe" to prevent splashing and possible build-up of static charge for storage of certain chemicals.

Furthermore, a bottom inlet allows for a shut-off valve to be mounted directly on the tank nozzle.... a preferred location

MJCronin
Sr. Process Engineer

 

[LittleInch]

My esteemed fellow posters I think have answered your question and I would agree with them.

There are few storage issues which are made better by having the incoming fluid pour from a high inlet or needing a pipe which can then syphon or create vapour conditions.

NRVs on tank inlets are quite common to prevent back flow on the inlet lines and provide a level of safety on not emptying the tank if there is a leak or breakage.

However I'm puzzled by your throwaway line "But the liquid tends to scale up over time"?? Liquids don't scale, but they can deposit scale on equipment ( valves, heaters, tank floor etc). what is the issue with scale and why does coming in a the bottom of the tank have anything to do with it?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

NRVs on tank inlets are quite common to prevent back flow on the inlet lines and provide a level of safety on not emptying the tank if there is a leak or breakage.

Perhaps "safety" is a poor word choice as most Hazop professionals generally do not like to rely on the use of rudimentary NRV's for safety.

http://jclrisk.com/wp-content/uploa...hor-Kay-Modi-April-25-2017-at-SynGas-Conf.pdf

 

[LittleInch]

OK, I agree that was not the best word choice, let's go for reduction in risk. I would also never rely on an NRV to do anything important and certainly never seal, but there is an expectation even in a HAZOP that it will reduce flow to 10% of what it would be if there was just an open pipe (though this may need two in series).

My point is that I think it is a common practice to install an NRV on an inlet line and one which is simple, cheap and in real life has a reasonable chance of working correctly. I wouldn't personally say it was "bad practice", but that's just IMHO.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[chicopee]

My question: is there a vapor space requirement or is the tank acting as an accumulator for the liquid?

 

[e43u8]

As per my experiences fire fighting storage tanks is an exception in which the inlet nozzle should be located near the tank bottom such that in any case the inlet line is being full of water.
I think the main reason for such arrangement is safety issue based on which in a pool fire case there is no chance for failure of the inlet line bringing the water to the tank...!