Why Suction Pressure Doesn't Rise in Test Loop?

12/16/2020

Some fire pump assemblies have closed-test loops. When we circulate water through this closed test loop, why does the suction pressure not increase?

I'd be interested in understanding the physics of the situation a little better here.

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7 Comments

Franck

12/16/2020 10:01:54 am

I hope that the water from the test line is delivered at the top of the tank and not on your pump suction side !

When you have a test line (very common in Europe, where we don't use test headers), the water is taken from the water tank (suction pressure is then related to height of water in the tank and possible friction losses depending on the length of suction pipe and flow demand) and the water is send back to the tank from the top (to avoid perturbation at the intake suction pipe).
In no way the return pipe from the test line should be connected to the suction pipe of the fire pump.

Franck

12/16/2020 10:23:57 am

Which means that with this installation, you are not increasing the pressure on the suction side (the water being delivered at the top of the tank has no pressure adding effect inside the tank).
The suction pressure remains the same, based on the height of water.

Colin Lusher

12/17/2020 01:39:10 pm

Frank, the closed test header loop as shown in NFPA 20 Figure A4.22.1.3(b) (2019) version is what the OP is talking about. This closed loop flows from the pump discharge directly back to the pump suction. We see these quite a bit in the US as we don't often use tanks for buildings that aren't high rises....we pull suction water from the public water supply.

Alex L

12/16/2020 02:13:32 pm

Apologies in advance for the long reply... If I understand the question correctly, the OP is asking about an arrangement that is very common in the United States- a “closed test loop” which is a piping loop with a flow meter from the discharge side of the fire pump to the suction side. The simplest answer is equilibrium. Bernoulli's principle for fluid dynamics states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid's potential energy. If that is not enough for you, we can break down some of the reasons why that is the case. A closed test loop is a fixed “system” with conservation of mass (there is no mass entering or leaving the system; i.e. the total mass of water is constant in the loop). With a check valve preventing more water from entering upstream of the pump, and no outlets in the fire protection system downstream of the loop, the water that we are boosting with the pump is the same water we are pulling from. Think of the system as two points, one on the discharge side (1) and one on the suction side (2), which are separated by the loop piping. There are two physical interpretations of the Bernoulli equation, one where energy is conserved (total head of the flowing fluid is constant along a streamline), and the other where pressure head and velocity head vary inversely. There are three terms in the Bernoulli equation, all related to head (if you remember, head is the concept that balances work and energy in a flowing fluid): Velocity head characterizes kinetic energy of the flowing fluid, elevation head is the gravitational potential energy of the fluid, and pressure head is the work done by the pressure force. Since the elevation of the two points (discharge side and suction side of the pump) does not change, we are left with pressure and velocity head. We control the flow through the flow meter by throttling the valve downstream of the flow meter, which allows more water to flow through the loop. Since the pipe diameter is constant, and flow is equal to velocity over an area (ft/s * ft^2 = volume/time), as we increase the flow rate we are increasing the velocity, which increases the velocity head term of the equation. This necessitates that the pressure head term must decrease. Remember that the combined head at (1) and (2) MUST be the same (in other words, the total energy of the loop must remain the same). One reason this occurs is friction! Remember that the Darcy-Weisbach equation for friction loss has several components: length of pipe, diameter of pipe, friction coefficient, acceleration due to gravity and velocity. The amount of friction loss will vary with velocity. TL;DR version: Another way to rephrase, when we flow more, velocity increases which results in more friction loss, keeping suction pressure constant in the loop. I hope that helps. (Source: my brain and applying concepts taken from the textbook Engineering Fluid Mechanics, 10th Edition)

Franck

12/17/2020 08:51:29 am

Many thanks for this extensive technical reply :)

As you understood from my answer above, we don't have this arrangement on fire pumps in Europe. And now I am quite puzzled.
But this is good as it keeps me learning things and this website is a good one for technical learning.

I therefore have some questions (if anybody can answer):

- Is this loop arrangement (discharge to suction) only installed on booster pumps or also on pumps taking suction from a tank ?

- What is the purpose of this test line ? Is it just for the weekly run of the pump (10-30 min) to avoid wasting water or avoid using the test header connections to create a flow and avoid an overheating of the pump casing at no flow (electric pumps)? Or can it also be used for the annual performance test on the fire pump (annual flow testing) as required by NFPA 25?

In the affirmative to the last question (annual flow test), how accurate can be the results from this test using the loop compared to using a test line going back to the tank or test headers?

Many thanks in advance for your feed back

ztsvi

12/18/2020 02:37:20 pm

This type of setup uses a loop of pipe which feeds back into pump suction. This means that in terms of energy, you effectively have the same elevation and mass flow rate throughout the loop. The energy input by the pump is in the form of pressure and heat.

To answer the initial question: the reason we don't see the discharge pressure at the suction is because of valves in the loop that we use to throttle the flow. The "simplest" way to do this is to just crack open one of the gate valves. This effectively creates and orifice plate that restricts flow (and pressure) downstream of the valve leading to the pump suction. The better way to do this is with a pressure relief valve or "waste cone" which is designed for this purpose. These devices are often included in the pump package from the manufacturer.

In terms of energy, the waste cone turns the energy stored as pressure into energy stored as heat. If left running long enough, this will become quite noticeable, and perhaps problematic.

Franck, to answer your questions: This loop is generally done to allow for the weekly/monthly tests so you don't waste water, so that you don't have to deal with draining large quantities of water, which can be messy.

During these brief tests, the water temperature will not increase very much, and instead the water flowing (instead of churning the pump) will prevent the pump from overheating.

Typically if you have a tank you would not need this loop, though there are situations where it makes sense to have both (particularly if your tank is on the roof and your pump is on ground floor - a return line to the tank would be more expensive than just putting in a loop).

This loop does not take the place of the annual flow test for NFPA 25, and so a test header must also be provided.

Franck

12/23/2020 10:24:07 am

Many thanks for the explanations

I understood this "strange" arrangement for the weekly/monthly run but couldn't see how it could work for the annula flow test.

You answer clarifies everything and thanks to you, I am bit less ignorant

;)

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