"how do hydrants are pressurised with 7 bar all the time even when the jokey pump is off"

We all know that water cannot be compressed, if compression is not possible, how can we maintain the pressure without an external force. we know that jockey pump is provided to maintain the pressure at 7 bar when ever it reduces, but when it stops the force or pressure exerted over the water body is also stopped, the water comes to it normal condition. (e.g.  take a sringe fill it with water, exert pressure over the water by pushing the piston, at the other end u can feel the pressure, as soon as u release the piston there is no pressure exerted over the water, now you will not feel the pressure at the other end).  Is there any compressed air provision attached to the hydrant system to maintain the pressure @ 7bar, if so please explain it.

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I have seen a hydrant system that is supplied via gravity. Which have huge ponds, and storage tanks that are elevated high above the city. This water is piped downhill to the water treatment plant. From there it is distributed to the rest of the city.

It has some serious high pressure on that line, which has no pumps. It uses jockey pumps or pumping stations for areas furthest from the water source (or areas that have elevation themselves) to increase the hydrant pressure to that specific neighborhood.

I see your an ARFF guy, me too. Assistant Chief Manchester-Boston Regional Airport, USA

Bill
www.fetcservices.com
thanks for u r reply, yes i am an ARFF guy. Airports Authority of India (Chennai Airport), but we use to pump it from static tanks which is below ground level. We do not use gravity to build up pressure in the hydrant system.
Along with what FETC said, the use of water towers also provides standard pressure on the water supply system. Again this is just a gravity feed and towers can be filled with a pump, but the height and gravity of the tower leads to adequate water pressure without the need for pumps.
Suresh, after reading your post again, your specific question is how to maintain 7 bar when pumps are off. In the hydrant system you have, (below grade with no gravity advantage) your system would need to have jockey pumps on, to maintain a certain level of pressure all the time.

Without pumps, your system would be negative, or what we call a dry hydrant statis source for which a pumper would have to draft (suck) water out of the piping connection.
Thanks for the reply, we do have jockey pump, main pump and a heavy large pump which almost all hydrant systems have, but in our buildings we maintain 7 bar in the hydrant system which also supports the sprinkler with the help of jockey pump. But my question is, in our pumping system we have set the pumps in automatic mode(i.e. when the pressure drops below 7 bar the jockey pumps starts to build up pressure to 7 bar as soon as it reaches 7 bar the jockey pump stops, if the jockey pump is not able to build pressure to maintain the 7 bar due to some reasons like any delivery opened at the hydrant point, the main pump starts, along with jockey pump the main pump builds the pressure upto 7 bar. After the hydrant is feached with 7 bar both the pumps stop. Approximatly atleast for 4 to 5 hrs if the hydrant does not have any leaks the pressure is maintained even if the pumps are in switch off condition. How is it possible? Any reasons behind it.
Sounds like it could be check valves right after the pumps so that the pressure cannot bleed off back through the pumps when they are off.
Thanks for ur replay.There are no check valves which will hold the pressure for water. Please give scientific reason.
First off, your theory is based on a syringe, which is not a closed system like a fire sprinkler/hydrant system. The only reason that a system can maintain pressure without a pump running, or gravity providing it, is a valve which closes once the pumps have brought the system to pressure and shut off, thus preventing the flow of liquid back to the supply side. This is normally a check valve of some kind. You might want to further study your system. This is just simple hydraulic theory, it is not that hard to understand.
Please clarify your question.

Are you talking about a municipal fire hydrant system? In many cases, in the U.S., these are fed by elevated tanks. It is not uncommon to have hydrants with 90 psi (6 bar) static pressure or more. Of course distance from the water tower and elevation changes can influence static or residual pressure. In other cases there are ground level water storage tanks that use pumps. I have seen systems in mountainous areas whose water tanks are on mountains near town. They will have great static pressure.

Are you referring to some sort of building or private water system? If so the pressure that the jockey pump achieves might be due to the number of floors the system pumps up. If you have a 25 story building whose system is simply filled with water, the pressure on the bottom will certainly be 7 bar or so. Many high-rise buildings have their own water tank on top. This will certainly account for a high pressure at the bottom. If you are in an area with a 7 bar pressure on the water system, that might account for the situation. We require our building fire protection systems to maintain about 60 psi on their systems. Our municipal water system usually provides that. But if it doesn't, their jockey pumps might come on.

There are many ways that static pressure can be set at a particular level and just maintained with pumps. Elevate tanks is just one way. There can be bladders or even the pressurization of the system. Yes it is true that water is virtually incompressible, but most materials can be slightly expanded to exert some kind of static pressure.

We need more information to answer your question well. BTW-In my world jockey pumps have little, or nothing to do with fire hydrants.

Generally speaking. “A jockey pump, or a pressure maintenance pump, is a small apparatus that works in conjunction with a fire pump as part of a fire protection sprinkler system. A jockey pump is designed to keep the pressure elevated in a fire suppression system so that the main fire pump is prevented from running unless absolutely necessary. It consists of a motor, a pump, and a controller.”
http://www.wisegeek.com/what-is-a-jockey-pump.htm

Are you calling a "Stand Pipe: a Hydrant?
Thanks for ur reply. I work for an airport fire service, in which the building consist of 2 floors only(i.e not more than 20 meters high) as per the ICAO (International Civil Aviation Organisation) standard the highest point of the building with the hydrant should have a pressure of atlest 5bar when tested and we do test it regularly and same is found. We donot have a high raised water supply, we just pump and fill the hydrant with water from the water source which is below the ground level, so there is no chance of gravity theory working on it because even the highest point consist of 5 bar pressure.

As per your quote u have mentioned that some materials can expand to exert static pressure, but as per our standards we use thick steel coted iron pipe in hydrant system which cannot expand to exert pressure, even though it expand it will not exert so much of pressure on the water body.

Lets try this: (Please even u try this in ur station):

Engage 2 length of hose to the delivery of a fire engine, fit a hand control branch which has a provision to check the pressure at that particular point, Now run the pump at 8 bar pressure, at the delivery end u can messure 7 bar pressure ( 1bar Approx., will be lost due to friction loss), now close the hand control branch to stop the water flowing out, even now u will find the pressure to be 7 bar at the delivery end.
Close the delivery out let while the pump is running, it makes the water cutoff from the pump which exerts the pressure to the water inside the hose. Now there is no force acting on the water body. Ensure that the hose doesnt have any leakage, check the pressure at the delivery end it should not drop below 7 bar for time not less than 1 hour. This time is taken because, the same as u said the hose will expand to exert the pressure over the water since hose is rubber lined.
What happen to the pressure????????
sorry my friend it not so as u think, if so then explain this.

We inflate our car tubes(With Tyre) with air, when it reaches or exerts pressure of 30 or 35Psi on the meter we stop filling the air, now the tube consist of 3 bar pressure(approx). Note:- The compresser should contain atleaset 50 or 60 psi to make the tube inflate to 30 or 35 psi.
Try to fill the water inside the tube(With tyre) with the above 3 bar pressure until the tube(With Tyre) is full or the volume it could hold. now check the pressure at the mouth of the tube will it show 3 bar pressure., no...
Air can be measured in pressure since it can be compressed, but water of its own doesnt have any pressure to exert since it cannot be compressed, Water pressure which is measured in our fire service is not the pressure of the water it is the force exerted over the water by means of some external force or gases or meachics.
Please give scientific reason.

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