Simple Hydraulic Loop - Cannot Figure out what the tank is for?

[Dersnerman]

Hello All,

I'm trying to figure out the reasoning behind what appears to be a bizarre system installation I just inspected at a plant. It's for an emergency eyewash loop. See attached diagram I sketched for it. To me it would all make sense IF the tank was a pressure tank (bladder tank), since this would provide for some backup pressurized water storage in the event of a power failure. To be honest I'm not sure if it is a bladder tank or not. When we shut the pump off, the pressure started to fall (as measured by the pressure gauge on the tank). We assumed since the pressure was falling, that it wastn't a pressure tank (otherwise pressure would have been maintained). However, now that I'm thinking of it again, we didn't leave it for that long before coming to this conclusion. It could be that the bladder tank set passive pressure was lower than system pressure while the pump was running. Anyways, please lend me your thoughts and opinions!

Bonus question - what is the purpose of the little recirculation line from the tank outlet back to the pump inlet?

 

[Dersnerman]

 

[pierreick]

Hi,
expansion tank/pressurized tank!

https://armstrongfluidtechnology.com/en/products/expansion-tanks

recycle line: pump protection in case of no flow out of the system.
Good design.
Pierre

 

[Snickster]

I would think that the tank is a bladder tank that maintains the system pressurized with water so that the instant any of the eyewashes are put into service you would immediately get flow rather than having to wait for flow due to pump starting, or if the pump does not immediately start, or does not start at all, there is enough water for one eyewash use. Also when the pump starts if without a bladder tank there would be a sudden pressure surge since the pump starts at minimum flow at highest pressure so the flow through the eyewash would be very high the instant the pump started up then taper off until the pump reaches steady state flow. In this way it maintains the eyewash flow more at a constant rate based on the pressure in the tank, not the startup pressure of the pump. You don't want a very high jet stream coming out of the eyewash when you first activate it.

Likely the recycle line provides for pump minimum flow requirements so as the operating eyewash station is shut down the pump keep running until no flow conditions when allowing bladder tank to be totally charged up to pump no-flow pressure conditions. Having a minimum flow also keeps the pump running at a point on the pump curve where the pressure output is withing the desired limits of the pressure at the eyewash so you don't get too much flow with one eyewash operating but enough flow and pressure with more than one eyewash operating.

Perhaps the bladder tank loses a little pressure when the pump is shut down due to pump discharge pressure equalizing with suction pressure if there is no check valve on the discharge?

 

[Snickster]

Noet that ANSI Z358.1 Emergency Eyewash and Shower Equipment, requires water flow availability for eyewash of 20 GPM for 15 minutes as a minimum. This would be what the net volume of a 500 gallon bladder tank would about provide if pump does not come on.

ANSI Z358.1:

4.1 Performance of Emergency Showers

4.1.1 A means shall be provided to ensure
that a controlled flow of flushing fluid is provided
at a velocity low enough to be non-injurious to
the user.
4.1.2 Emergency showers shall be capable of
delivering flushing fluid at a minimum of
75.7 liters per minute (20 gpm) for a minimum of
15 minutes. If shut off valves are installed in the
supply line for maintenance purposes, provisions
shall be made to prevent unauthorized shut off.
4.1.3 Emergency showers shall provide a
flushing fluid column that is at least 208.3 cm (82
in.) and not more than 243.8 cm (96 in.) in height
from the surface on which the user stands.
4.1.4 The spray pattern shall have a minimum
diameter of 50.8 cm (20 in.) at 152.4 cm (60 in.)
above the surface on which the user stands, and
the center of the spray pattern shall be located at
least 40.6 cm (16 in.) from any obstruction. The
flushing fluid shall be substantially dispersed
throughout the pattern.
4.1.5 Emergency showers shall be designed,
manufactured and installed in such a manner
that, once activated, they can be used without
requiring the use of the operator’s hands.
4.1.6 Emergency showers shall be
constructed of materials that will not corrode in
the presence of the flushing fluid. Stored
flushing fluid shall be protected against airborne
contaminants.

 

[Snickster]

Just noticed that the main line to showers provide a recirculation path too so not sure about the line just downstream of the tank with valve shown as normally closed.

 

[LittleInch]

I'm with the others here that it looks like it should be a bladder tank. Is there no markings or labels? Also what pressure are you measuring ( though it will be the same until it empties), the bladder internal pressure or the water pressure?

The hardest thing to check is whether the bladder is still intact. The only way is to turn off the water source (what pressure?) turn off the pump and then open a shower or that spare connection and see what happens with pressure and flow/volume. If it falls like a stone then your bladder might be no more, but if it slowly reduces in pressure but continues flowing then you're prob ok. If the pressure stays at some pressure and the flow stops then the PG is measuring the bladder pressure at empty.

If there is no NRV on the pump then the water pressure will equalise throughout the loop hence the reduction in pressure you see.

That recirc line? Loos redundant, but maybe the shower recirculation line was added later.

Running the pump 24/7 seems a bit of a waste?


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

 

[Dersnerman]

Thank you all for your quick and thorough responses! I have some further investigating to do then. I agree with most of your suspicions on this.

Purpose of bladder tank - alleviate surge and provide pressurized water storage during power outage.

Purpose of pump - Boost pressure of incoming water line from approx. 50 psi up to 80-100 psi for shower flows. (I assume eyewash stations each come with a pressure reducing valve to limit the spray?)
Note that pump could either run continuously or start upon detection of low pressure via simple pressure switch similar to residential well installation.

Purpose of recirc line - still a bit unclear here. I agree with Snickster that it could have something to do with maintaining pump flow rate in optimal point on the curve during various operations, however, wouldn't the primary loop (for the eyewash stations) serve the same purpose?

 

[Snickster]

Looking further that system does not make sense on a few levels.

With bladder tank pump should not operate continuously but should start on low pressure after stored volume in tank is used so continuous pump operation makes no sense.

Minimum pressure required for safety shower is about 30 psig so I believe the line returning from the showers should be designed so flow from the water supply can flow directly to the showers when 50 psig is normally available. This may be the line you refer to as recirculation line because the line coming from the showers to the pump must have a restriction orifice in it as described below.

I don't see why you would come back all the way from the showers to the suction of the pump. If it was meant to be a continuous circulation line then you could but a recirculation line more local to the pump. If it were really for pump recirculation then it needs a restriction orifice to limit flow to pump minimum flow or the pump would not develop any differential head much above the water supply pressure because it will always operated at end of curve without any restriction so the pump will be useless.

 

[pierreick]

Hi,
On a safety point of view, not knowing where your facility is located, I've a concern about your eye wash system. It's about the temperature of the water. Normally the recommendation is to use the eye wash for at least 15' to get good result. This is impossible if the water is cold (winter), the reason why we installed a heat exchanger on the loop to get tempered water suitable for eyes wash. You should discuss with your nurse/doctor at your facility. The drawback of tempered water could be the growth of bacteria in the system, regular check is required.
Note: same comment for safety showers.
My comment is based on a similar unit installed in China where we were using phenol and caustic soda as Raw mat.
Pierre

 

[Dersnerman]

Thank you both for your continued support on this topic.

Replying to Snickster, I agree the pump should not run continuously. My suspicion is there is a problem with the bladder tank, and the system has been modified from its original purpose. Also good call on the recirc line. Perhaps it's not a recirc line at all but a bypass feed to the eyewash/showers when the pump is not available. Re: orifice, there may be an oriface or there could just be so much head loss through the long piping system that enough headloss is generated for the pump to operate on an appropriate point on its curve.

Replying to Pierreick, yes this is also something I was thinking of. I need to do some further investigation to determine if any water heaters are currently in place.

 

[LittleInch]

The other aspect though is all that energy from the pump in constant recycle will warm up your water, but to an unhealthy temp. Beware legionnaires bacteria. Systems need to go to 60C on a regular basis.

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

 

[Dersnerman]

For the temperature issue, I would think having on-demand water heaters near the eyewash station(s) could be a good solution to this. They take a few seconds to warm up but otherwise the water would stay cold under normal conditions.

 

[Dersnerman]

Alternate thought / solution:

The tank is in fact just a pressure vessel (not a bladder tank). It's purpose is to simply let the water come to room temperature (therefore tempering the water somewhat at least, to about 20C). Pump is for recirculation only, so the water is not stagnant. Agreed the pump needs some backpressure to operate efficiently on an appropriate point on the curve. The "recirc" lines is just a bypass around the pump and tank, in case there is an issue with one of those, water can still flow from the source to the eyewash/showers. Albiet colder water, but still have water.

 

[Snickster]

I was also thinking that the pump was just for recirculation to keep water from being stagnant. However you did not give any flowrates or pump size or line sizes. If you had better information it would be easier to figure out the system. Like if it is a bladder tank it must have a manufacturer's logo on it somewhere, and pump must show a model and horsepower on name tag, and you said pump raises pressure from 50 psig to up to 100 psig but if recirculation only this cant be correct, and if only recirculation then pump must be very low horsepower with small inlet and outlet and return line from showers must be small diameter too, unless it is large diameter which would indicate it is then the main supply to the showers from the water system. It is not possible to figure out the system not knowing any of this information.