Basic question about flow and pressure convention 1

[JH2015]

When a domestic water supply to a building is specified as flow and pressure available at a tap of 1 bar and 1 l/s for example, does this mean

A) 1 bar behind tap when closed, and 1 l/s out of tap when opened
or
B) 1 l/s available when tap is opened, with an added headloss of 1 bar?

Unless I'm missing something, the pressure at the outlet of an open tap is always 0 barg (without an added fitting to create a backpressure).

Please see attached sketch.

Many thanks

http://files.engineering.com/getfil...-b49b-910862511302&file=flow_and_pressure.png

 

[IRstuff]

Static pressure in a water line is typically much higher than 1 bar, isn't it?

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[JH2015]

The actual numbers are made up for the sake of illustrating the point...

 

[BigInch]

That would be inside the pipe, before nozzle, or discharge coefficient loss. Pressure is 0 Barg just after the pipe terminates, in free air, where the fluid stream is no longer confined by a pipe.

 

[LittleInch]

In your example it means 1 bar at the point where the pressure guage is shown at 1 litre/second. It would tend to assume that the tap or valve is not fully open and hence some throttling is occurring, e.g. a restriction in the tap or the tap isn't fully open or it is feeding something else which has a convoluted or small set of pipes, e.g. a dishwasher or washing machine.

When it is expressed as pressure at a flow this make sit clearer, but otherwise it wouldn't make much sense.

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

 

[bimr]

It means that when the water is flowing out of the service, that the water will be flowing at 1 l/s and at a pressure of 1 B. This assumes water is flowing into your system in a confined pipe.

Think of it this way, the water main is typically 8-Inch diameter and there is minimal headloss across the service valve, so the minimum pressure in the water main is 1 B.

The municipality generally provides a minimum pressure of 1.4 B during fire flow scenarios.

 

[CostasV]

If someone specifies flow and pressure, then the location of the pressure gauge is a major factor ans should also be specified, which if it specified it will make things more confusing (the OP is an example) .
IMHO specifing both flow and pressure, especially for domestic water supply, is wrong. Maximum flow (with fully open valves) is what matters.
Pressure should be specified only for selecting the right materials and the leaking or strength tests.

 

[jgailla]

For a system curve, pressure is meaningless without flow and flow is meaningless without pressure.
In your case, you might have 2 l/s at 0.5 bar or 0.5 l/s at 2 bar. One point doesn't tell you anything useful.

 

[bimr]

People are missing the big picture.

Municipal utilities generally have the following responsibilities for maintaining water pressure:

•Maintain minimum operating pressure of 35 pounds per squire inch (psi) throughout the water distribution system.

•Maintain minimum operating pressure of 20 psi under fire flow conditions.

When you a install a service tap of 1.5-Inch into a municipal 8-Inch water main, you are going to receive the 1 l/s water flow without affecting the utilities water pressure. If you were installing an 8-Inch connection to the utility, that would be a different story.

The OP is responsible for designing the system from the service tap to the building.

 

[JH2015]

Thanks for all the replies. There are obviously lots of different ways to talk about flow and pressure, but I wasn't sure if there was a standard convention amongst plumbers. I'm sure I've seen a plumber measure the pressure at the outlet of a tap by blocking the flow, and it struck me as a meaningless measurement, as a high pressure is useless if it doesn't correspond with a decent flow rate. Likewise, a good flowrate from a tap is not sufficient if there is not sufficient pressure to maintain a good flowrate with the addition of something like a showerhead or washing machine.

Anyway, for my purposes, I've installed a pressure gauge immediately upstream of the tap, partially closed the tap to restrict the flow to 1.2 l/s, which generates a pressure of 1 bar at the gauge. I can then tell them they've got 1.2 l/s at 1 bar. I.e. they could get 1.2 l/s up to a height of 10m within their building (minus any other losses they add obviously).

Thanks

 

[bimr]

Plumbers are not water supply experts or plumbing engineers, they are pipe installers, so you should not expect an answer from a plumber. What is more important is what is on the other side of the water main, a water system with 100 MGD capacity or a small water system with low capacity.

Regarding "I'm sure I've seen a plumber measure the pressure at the outlet of a tap by blocking the flow, and it struck me as a meaningless measurement, as a high pressure is useless if it doesn't correspond with a decent flow rate."

It is more meaningful to understand what you are doing. If you are tapping an 8-inch water main with a 1-inch service tap, it is pointless to do a dynamic water pressure test as the 1-inch service tap is not going to affect the available pressure in the 8-inch main. It doesn't matter whether the water is flowing or not.

If you are tapping a 2-inch main with a 1-inch service tap, the service tap may affect the available pressure because the 2-inch water main does not have the large flow capacity as an 8-inch water main. In that case, you would want a dynamic water pressure test.

On a municipal water supply system the dynamic water pressure and flow seen in a building will drop to a number lower than the static water pressure but will normally remain steady when you turn on one or more plumbing fixtures.

Regarding "Anyway, for my purposes, I've installed a pressure gauge immediately upstream of the tap, partially closed the tap to restrict the flow to 1.2 l/s, which generates a pressure of 1 bar at the gauge. I can then tell them they've got 1.2 l/s at 1 bar. I.e. they could get 1.2 l/s up to a height of 10m within their building (minus any other losses they add obviously)."

This statement is incorrect and meaningless because if the pressure is only 1 Bar at the ground level service tap, the water will not flow out of the tap at a higher elevation of 10 meters. You would also want to simulate the water use in the residence such as opening two faucets at the same time, not cracking a valve at some unknown percentage.

Municipal water systems in general are required to have a minimum pressure of 1.4 Bar during fire flow scenarios.

 

[JH2015]

"This statement is incorrect and meaningless because if the pressure is only 1 Bar at the ground level service tap, the water will not flow out of the tap at a higher elevation of 10 meters."

OK, but please explain what's wrong with this logic then:


1 bar is measured upstream of a partially closed tap, at a flow of 1.2 l/s. Immediately downstream of the tap is a free discharge. Therefore the pressure loss through the tap is 1 bar at 1.2 l/s.

If you then remove the tap, and replace it with an elevation change of 10m, the pressure drop through this elevation change is also 1 bar, but consists of static head rather than friction head.

Therefore, the 1 bar that was pushing 1.2 l/s through a partially closed tap is instead now pushing 1.2 l/s up 10 m (ignoring the added friction loss of the increased pipe length and fittings, so I appreciate in practice you would maybe only get the water up to 9 m or so, depending on the pipe diameter).

 

[bimr]

You would want a minimum of 1 Bar pressure at the faucet at the 10 meter level. Therefore, you would want to see 2 Bar at the service tap.

When the water is running, the pressure gauge is measuring dynamic pressure, not static pressure.

If you want to determine the actual pressure that someone in the household will experience:

Connect two faucets to the service tap and measure the pressure. The faucets will provide a real world simulation of the actual flow and pressure conditions.

1.2 l/s is more flow than the typical residence (faucet) would use and you have introduced a partially closed valve with an unknown pressure drop. You do not know what the pressure will be at the 10 m level.

 

[JH2015]

It is not a residential application. The partially closed valve had a known pressure drop as I measured it - 1 bar upstream, free discharge immediately downstream, therefore 1 bar pressure drop at 1.2 l/s. The tap was to allow me to measure and specify a supply flow and corresponding pressure.

I shouldn't have mentioned the 10m, it seems to have confused things. It was merely meant to illustrate the meaning of a flow at a pressure, not to represent an actual application.

 

[bimr]

Okay, so you have proved that you have plenty of water flow capacity, but you still do not know what static and residual pressures are available in the water main.

You have 2 choices to determine the system pressure:

1. Install a flow meter. Open the valve to obtain the desired (expected usage rate) flow rate. Measure the pressure. That will provide the flow and pressure at your application rate

2. Contact the water supply utility. The water supply engineer will review the water network and provide an estimate of available flow and pressure. Most utilities will do a hydrant test:

https://www.aspe.org/sites/default/files/webfm/ArchivedIssues/2011/201112/FocusonFireProtection.pdf

How to do a pressure test:

http://www.twsuk.com/16_how-to-do-a-pressure-and-flow-test