Pump design for a safety shower system 2

[villivord]

Dear all,

I'm currently designing a centrifugal pump to boost the pressure in an operating safety shower system. As I'm relatively new to pump sizing, I have some technical questions.

The current system consists of 3 safety showers/eye baths located on three different floors of a 100ft (height) building. The booster pump shall be located at the ground level. The current pressure at the ground level varies between 52psig and 65 psig. They told me the pressure should be 75 psig. The static difference between the pump's centerline and highest shower head (located on the highest floor of the building) is 82ft. The friction losses in the piping going to the highest safety shower requires 15 psig extra head. The total head required by the pump should be then 51 psig in total (static diff. + friction losses). Can I assume then that the pressure drop across the safety shower/eye bath is 24 psi then? Otherwise you have more pressure then the system requires so I would assume then that your operating point shifts to the right according to the centrifugal pump curve.

When I read on the internet about safety showers, they state for example that the working pressure of a safety shower is between 29 psig and 102 psig. Is this the pressure drop across the safety shower (depending on the flow rate) or otherwise said, the required pressure directly upstream of the safety shower?

KR,
Villi

 

[ashtree]

What i would do is design for the safety shower at the highest elevation and use the lowest feed pressure which looks like 52psi.
You have calculated your friction losses so i assume you know what the flow rate to your shower is.
The working pressure is the pressure required to deliver the required flow through the valve , pipework and shower so you will have to determine what that is for the showers you have installed. Ask the supplier if you don't have the detail otherwise you might have to do some testing yourself with pressure gauges and some way to measure the flow. This operating pressure will have to be added to the pump head. So the head will be this: Pump Head= (static+friction+shower pressure)- service pressure.

A couple of things to think about.
1) What is the risk of service pressure being below 52psig. If this is a significant risk maybe you need to consider a bit of a margin on this.
2) What is the likelihood and the consequence if two or maybe even three safety showers had to be operated at the same time. If this can never happen than your approach is correct. But if there is a chance that this could happen, then you will need to consider a larger pump, to meet the greater flow requirement and increased friction loss.

Maybe a multi pump set up might be useful to cover this scenario and it would also give a degree of redundancy.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[EdStainless]

You could look at a system where you pump up to an accumulator tank at the top floor, and then have individual regulators at each of the lower locations. This is how water systems in tall buildings are often designed.
You need to know min required pressure at the shower and what the flow will be.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[chicopee]

EdStainless has the right approach; you need a storage tank with enough capacity for three showers; the storage tank should have a minimum water level once showering with three showers running simultaneously, is done. The max level is to be based on the estimated amount of water needed plus some free board.

 

[chengg29]

Villivord,

Before you look at anything else, I would reckon that with all the system information that you have with you viz. buliding height, static head friction losses etc. you should be able to draw a rough sketch of the entire pumping system with the 3 safety showers located at various levels. In this sketch show the most plausible routing of the pump discharge pipework to all the three destinations with the necessary bends,tees, elevations etc. of each of the three discharge piping up to their individual destinations viz. safety showers. Then do the pump calculations for each individual pipe network and then analyse against them. You may have a much better assessment in front of yourself.

 

[villivord]

Thank you for the replies.

@ashtree: I forgot to mention the flow rate (22,5 gpm) I used for the calculations. The current safety shower system is very old so no information regarding working pressure of the safety showers is available. We also assume that the likelihood of two showers being used at the same time is too low in our case so the flow rate should be sufficient.

@chengg29: I made such a drawing and I used the route with most losses (piping length, bends, static height) to determine the required pump head.

 

[EdStainless]

You are going to need regulators in the system even if you don't use a storage tank.
If you provide 75psig to the one on the upper floor that would be roughly 120psig to the one on ground floor.
That is why they usually pump to the top and then feed down, design for the upper and see where along the way you need pressure reduction.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[villivord]

@EdStainless: I made some mistakes in my first post, sorry for that. It is the booster pump's discharge pressure which should be 75 psig. This means for my case that the required pump head shall be between approximately 10 and 23 psig depending on the service pressure. This means that no where in my system, the pressure shall be above 75 psig, right? The pressure at the lowest safety showers shall be indeed higher but are regulators still necessary then? The design pressure of the piping is 125 psig.

 

[LittleInch]

Villiers,
Before you go off designing a pump you need to get your system design requirements sorted out. Now you tell us someone wants 75 psi, but earlier you said you don't know what each safety shower pressure needs. So how did anyone come up with 75.
Is this the min pressure, max pressure what?
Why is a booster pump suddenly required of the showers are so old no one knows what pressure range they need to work properly? ??

You use head and pressure as the same thing. It isn't. Head is metres or feet of liquid, pressure is psi or bar. You're an engineer, use the correct terminology.

Pressure for the same head varies with density. At the moment you're using water but another time it might be something less dense.

You have two variables here which makes getting the same pressure at each shower difficult, namely variable inlet pressure and a height difference. Hence you need to find out if a fixed speed pump will be acceptable by looking at the max pressure on the lowest floor with the highest inlet pressure vs pressure on the highest floor with the lowest inlet pressure and the same differential head from the pump.

Only then can you decide on the type of pump or pressure control system.

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

 

[villivord]

Thank you for the response LittleInch. The problem in my case is that it was already designed by someone before me (I'm not able to contact this person regarding details of his calculations as he left the company) so I'm trying to understand his design. To make things clear: it is the pump's discharge pressure which has to be 75 psig. The required head will be then between 2.3 ft and 5.2 ft (my apologies for the disturbance with psig) depending on the service pressure. The pump is a frequency controlled unit (VFD) so it will adjust it speed depending on the measured discharge pressure. I recalculated the pressure losses (46 psig) for the specified flow rate (22.5 gpm) so I believe a working pressure (for the safety shower) of 29 psig is assumed here. In my initial post, I was looking for answers regarding the working pressure of safety showers, but this is confirmed now by previous reactions so 29 psi seems a realistic value according to me.

Another question I have: In case of pump failure, is there a possible scenario of going to shutoff pressure as most of the time, the showers are not operated (so no flow)? During normal operation (proper working pump), the discharge pressure is set so it will control itself to not exceed it and keep the system pressure constant.

 

[LittleInch]

villivord.

Please next time can you include all relevant information in your OP rather than drip feeding data into the thread.

I'm not sure if you grasped ft water and psi yet. 1 psi is equivalent to about 2.3 ft water.

You min and max psi difference across the pump is 10 psi to 23 psi, equivalent to 23ft to 53ft. You were a factor of 10 out, but that's quite a big head difference from one extreme to the other.

So using your previous data the worst case is your top floor - I get a total of 50 psi pressure loss (82 ft head gain plus 15 psi friction losses) so its about 25psi at the shower fitting - minimum previously quoted

the highest pressure depending on your friction losses could then be around 65-70 psi at the ground floor. Now that's not much more than it can see now so shouldn't be an issue, but do these things get tested?

I suppose my biggest concern is that this is a safety shower, i.e. one that, hopefully, should never be used. To fit a VFD pump for such a duty and then let it sit there for a very long time not being used doesn't sound like a good idea to me.

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

 

[villivord]

LittleInch, Can you please explain in more detail why this doesn't sound like a good idea? The only thing I can think off is the fact that these VFD booster pumps are typically placed in systems where there is a strong demand (for example a plumbing system), which is not the case for safety showers. Are there other/better solutions here (pump type, ...)? Furthermore, what pressure will I read on a pressure gauge placed upstream of the safety shower (really close to the shower)? Is this the discharge pressure minus static and friction losses (except for the losses due to the safety shower itself)?

 

[LittleInch]

Because the system is sitting there not in use for a very long time so when it needs to be used could easily be switched off, broken, seized up......

Yes the pressure guage immeadiately upstream the shower will be discharge minus static and friction when the there is flow. No flow, no friction losses.

Some safety showers in situations like this basically have their own tank and use a much lower pressure.

Like this

http://www.directindustry.com/prod/hughes-safety-showers/product-15889-44778.html

then you can trickle feed the normal supply in.

the ones on the ground floor and middle floor would seem to be ok just with service water - it's the top floor that is the problem.







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

 

[villivord]

In case of pump failure, for example: the pump is not able to control the discharge pressure anymore. In that case, I will read a higher pressure on the pressure gauge due to the fact that my pump will go to shutoff head or is this not correct?

Thanks for the link, I have never seen this type of safety showers before but it seems an option to consider in my case

 

[ashtree]

Just to clarify a point, and maybe i do not understand what you are saying but shut off head is the head at which the pump when running can no longer generate a flow. Shut off head is at the extreme left side of the curve, the point where there is no flow.
If you have a valve on a pump discharge and you gradually close it with the pump running, the shut off head is the head on the discharge side of the pump when the valve is fully closed.

Shut off head is not the system head when the pump is shut off.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[villivord]

@ ashtree: As the safety shower will only be operated in case of emergency, there will be no flow during standby operation so the pump will go to shutoff head if the intrinsic pump pressure control should fail? Is this a possible scenario?

 

[ashtree]

If the controller failed causing the pump to run whilst the showers are turned off , then the pump would be in the shut off condition.
It would be most likely that if the controller failed then the pump would not start or stop if it was running. However it is not impossible that it might continue to run.

I agree with Little Inch's comment about a VFD on a safety shower pump system. Because this is a safety system you want the whole thing to be as reliable, robust and as simple as possible. Therefore you would choose a DOL starter if possible, and if the pump supplied slightly more flow than was necessary or ran at a bit higher pressure because it was not as accurately matched as what a VFD control might allow, it would make almost no difference for the little bit of work that this system would do.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"