Pumped Network with Pipes Lower than Source Tank 2

[melkordy]

Dear All,

I have searched this forums but couldn't find a thread that describes what am facing exactly so I started this one.

I have a pressurized piping network that is being fed from a pump station and a reservoir that is higher than most of the pipes of the network (the pumps are required in order to deliver extra pressure at withdrawal points and to overcome friction).

When the pumps are not in operation, there is a possibility of having the source tank drain by gravity through the faucets as it is higher than most withdrawal points.

In order to prevent that, I have suggested to force a gooseneck at the pump station just downstream the pumps check valve and the hydropeumatic tank. This is by raising the discharge pipe to a level of 50cm higher than the water level in the tank and place an air valve (vaccuum breaker) at the high point before going down again to the original level and connect to the network.

When power is turned off, flow from the tank (without the pumps in operation) will create a syphon which shall be broken by the vacuum breaker valve and stop the flow.

Is this solution feasible? Is there another alternative?

What precautions shall I consider while designing surge protection for the network (against power failure)?

For power failure, I have placed a typical hydropneumatic tank just downstream the pumps (a bypass line will not work efficicently as the source tank is not much higher than the pump). The problem is that the hydropneumatic tank shall drain completely to the network after power failure. Is using bladder type surge tank more suitable in that case? Also, negative pressures form in the network at high points when the pipes tend to drain by gravity through faucets located at the low points. If I place vacuum breaker valves at thse high points I shall facilitate draining of the network after power failure; Is this acceptable?

Any help will be much appreciated, this is the first time to deal with such situation.

Thanks in Advance
Mohamed Elkordy, M.Sc.

 

[bimr]

Is this solution feasible? Is there another alternative?

Have you considered the installation of an energize to open fail close valve in the suction line to the pump? One would think it unwise to rely on the gooseneck as you would have to carefully design the pressure conditions under which it would work. The pressure conditions consist of very low head and velocity head and will be difficult to make work at such conditions of low pressure.

You should have an emergency generator for the power failure scenario such that you have a minumum water distribution system pressure of 20 psig at all times. It is not good practice to use vacuum breakers as you will be required to disinfect after losing pressure in the water distribution system.

 

[melkordy]

Dear Bimr,

Thank you for your response,

What do you mean by "an energize to open fail close valve in the suction line to the pump"?

I think I will have air into the system any way when I loose pressure (when power fails) as there are high points in the network with air valves installed. they shall help draining the lines to the faucets.. I know that this should be an emergency situation but I am afraid of loosing all the water of the hydropneumatic tank as well.. that's why i need to install something to shut the flow down when power fails. This will prevent the source tanks from draining, prevent the hydropneumatic tank from draininf completely, and also prevent having water flow through the vanes of the pumps when they are not in operation..

what do you think?

 

[bimr]

Just a valve that will automatically close when you shut the pumps off.

 

[Pumpsonly]

The goose neck with the auto air valve will work to break the siphon and prevent draining the source tank. You need to elaborate more on the operation details as when do you need the pump on / off.
Is it manual or auto?
If water is continue being drawn from the system while the pump is off, you may face a situation where the pump is started with an empty pipe line that is -ve static head. If the flow velocity is high you will face a water hammer situation. You will need to size the air vent valve correctly to avoid too fast filling.

Anther scenario is the air vent valve will not work if the goose neck has full flow during pump starting.

You did not gave detail of the pump flow rate and pressure and also the piping system. If the flow velocity is low, surge will not be an issue.

 

[melkordy]

Thanks pumpsonly for your reply..

Operation at the pump station is automatic.. I have 6 pumps operating in parallel during period of high demand (total flow of 330 lit/sec and velocity 1.6m/sec) and this number decreases with demand decrease according to the pressure.. when no flow is being withdrawn, the network should be kept pressurized...

The situation of loosing pressure at the pump station is an emergency situation when power fails at the station (during period of high demand which is more critical).. In this case, water still flows through the faucets and the network could drain completely. I thought of the gooseneck with air valve to prevent the source tank from draining too...

We shall put precautions on startup of the station when power is back to fill in the lines slowly.. However I don't know how can this be done (the pressure is now almost zero at the pumps and all 6 pumps should be triggered to operate once power is back)..

 

[Lan123]

As bimr suggested control valve is a definite option. If the purpose is to prevent the tank draining, an electronically & hydraulically activated control valve just downstream of the tank, but before the distribution system will do the job. For the hydraullicaly activated valve I would set the target point few psi's below the current minimum pressure at the proposed installation location but would maintain more than 20 psi at the highest point. For the valve, I would talk to CLA-VAL. They will provide you valve that would meet your requirments.

 

[bimr]

Not sure what application consists of, but the flow rate that you have proposed (330 l/s) is probably too large for a hydropneumatic tank.

 

[Pumpsonly]

Unless a anti-surge system is in place,installing a energize to open valve at the suction to prevent drain out the source tank in the event of a power failure is a sure way to induce water hammer in the system unless the closing of the valve is controlled to allow continued water supply to the system until the fluid velocity in the pipe is slowed down to a safe value.

The air release valve can be sized to allow a controlled venting of the air based on the pump capacity and empty pump volume. Reputable valve manufacturer will have their own sizing method based on their valve design.

The air valve also helps to prevent water hammer in the event of power failure by admitting air into the system.

 

[melkordy]

I shall use the air valve next to the hydropneumatic tank or I can use dipping tube vessel.. I am much comforted to the gooseneck solution than the pump control (energize to open) valve.. at least it is more assured to work

 

[bimr]

Maybe pumpsonly will be able to provide references where he has such systems in place.

You may be comforted, but an air release valve will simply not work properly at the near atmospheric pressures that you will encounter. There will not be adequate head to close the air release valve. Air release valves are not designed to operate at 1-2 psig (approximately atmospheric head).


The air release will definitely cause water hammer because it will allow a phase separation to occur when air is introduced into the pump and pipeline. The introduced air will also defnitely destroy the pump(s).


I fail to understand why any responsible person would ever want to introduce air into a centrifugal pump. That will destroy a centrifugal pump quickly! Here is a reference:

"An air locked pump will overheat in a matter of minutes."

Refer to page 8-18 of this document:

http://www.ragsdaleandassociates.com/WaterSystemsCertificationManual/Chapter 08 - Pumps.pdf

This application is also way too large to use a hydropneumatic tank. Hydropneumatic tanks are normally used on applications with flows of 3 l/sec not 330 l/sec.

All automatically operated valves come with solenoid venting control devices that allows the valve closure time to be adjusted. Here is an example of inexpensive devices that allow slow closing of valves:

http://www.lesman.com/unleashd/catalog/valves/valves_8221.html

Probably the least expensive method is to get a check valve that includes the automatic closing option. See the link below:

http://www.controlvalves.com/series/94/94.html

The Series 94 is designed to open wide as long as inlet pressure exceeds discharge pressure. Should pressure reversal occur, control tubing in the valve carries back pressure to the bonnet forcing the valve closed. Also available as the 94-1 Check With Opening Speed Control; the 94-2 With Closing Speed Control and the 94-3 With Opening and Closing Speed Control.

This is your best option to prevent water hammer, siphoning, and destruction of the pump.

 

[Pumpsonly]

bimr,
Although I am new to this site,I have been in the pump industry long enough to know that introducing air into the pump in a NO No. You may want to read again the original post that the proposed goose neck with the air valve is install at the down stream ( discharge side)of the pump. Not at the suction piping.
This probably what led you to say the valve will not work near atm. pressure.

Phase separation in a pipe line is only harmful if it create a vacuum in the process and cause the separated liquid columns to collide again.

I am not a water hammer expert, but I do know what causes water hammer or pressure surge in a pipe line when the pump is suddenly shut down in a full flow.
The pump internal resistance slow down the flow of the water from suction while the water in the discharge pipe continue moving forward due to its momentum and a phase separation occurred.
Installing a auto closing valve at the pump suction line will further worsen the problem.
One of the old method to mitigate the phase separation is by increasing the inertial of the pump and driver by adding flywheel to prolong the coasting time of the pump.

You may also want to take a look again how the series 94 valve work. Are you still proposing it to be installed and how at the suction piping and how can it prevent the draining of the tank when the pumps are stopped?

Melkody,

As I suggested earlier, you may want to ask the moderator to move the whole thread to the pump engineering forum if posible to get more response.

 

[bimr]

This is not a pump question. This question has to do with the operation of a system including the pump, the piping, the reservoirs, and the control system.

One should be aware that introducing air into a pipeline is also a No No and is usually avoided. The air will eventually be trapped in the pipeline at high points because the pipeline would never have been constructed with no slope to it.

In addition, if you have 2 phase flow (air and water) from the introduction of air, the capacity of the water pipeline will be reduced.

The air release valve is also not a positive method of preventing the siphoning. The original post calls for a "vaccuum breaker". Since there is pressure from the upstream reservoir, the pressure at the location of the "vaccuum breaker" may not get low enough to actually activate the "vaccuum breaker".

These hydraulicly operated valves made by Cal-Val, OCV and others are intended to:

1. Slowly open after pump start;
2. Slowly close at pump shutoff; and
3. Close rapidly as a check valve in the event of power failure.

These valves are mounted on the discharge in the place of standard check valves.

The valves act as check valves;
The valves prevent water hammer;
The valves prevent siphoning through the pump.

Read further what the valve will do. This is the linke to the correct valve:

http://www.controlvalves.com/literature/JVGrL-KRXcD-IxXty-ocvmanual_series125_3-1-2007.pdf

http://www.controlvalves.com/manuals/ylQqK-JyfXP-UFDne-125-27P_Pump_Control_Valve.pdf

To accomplish this, the valve is electrically interlocked with the pump motor and equipped for controlled opening and closing in coordination with the pump. The 125 installs on the main discharge line of the pump.

By the way, to prevent water hammer damage when the pump power fails, the check valves are supposed to close rapidly, not slowly.

 

[Pumpsonly]

bimr,

1) Air Valve
If you read again the OP stated that the source tank and pumping station are both higher than most of the draw off point and water will continued to be drawn from the pipe line by gravity after pump tripped until it is empty. He will need to install air valve at the pumping station to release the air during restart.
He also mentioned that there are already air valves installed at the high points in the network.

2)You mentioned in your 2nd May reply the the valve is to be installed in the suction line to the pump.That was why I found your reply questionable.

3)

In addition, if you have 2 phase flow (air and water) from the introduction of air, the capacity of the water pipeline will be reduced.

When the pumps are running,the pump pressure will keep the air valve closed after venting of the air in the pipe line. How can 2 phase flow condition occur?

4)

The air release valve is also not a positive method of preventing the siphoning. The original post calls for a "vaccuum breaker". Since there is pressure from the upstream reservoir, the pressure at the location of the "vaccuum breaker" may not get low enough to actually activate the "vaccuum breaker".

The OP stated the goose neck will be above the water level of the tank.Once the pumps are stopped, water continued to drawn from the pipe line due to gravity flow. The vacuum created will allow air into the pipe line and break the siphon.

http://ventomat.com/siteimgs/air.pdf

5) Hydraulically operated check valve.
This type of valve make use of the down stream pressure to force close the valve seat. In this application, the down stream side will be in a vacuum condition if air is not admitted into the system while the up stream will have the static pressure from the tank.

Do you still think the valve will work as it suppose to?

6) [quote) By the way, to prevent water hammer damage when the pump power fails, the check valves are supposed to close rapidly, not slowly. (quote}

The fundamental of surge alleviation in the pipe line during a power failure trip is to ensure continue supply of liquid into the pipe line as long as possible to prevent or delay the formation of liquid column separation till the velocity is at it lowest possible to minimize the surge pressure. The surge vessel and fly wheel method typically work on this principle.By closing the valve immediately will cause the liquid separation to happen sooner and at higher run away velocity hence higher impact when the separated liquid columns rejoin.
The check valve are suppose to close when the forward flow ceased
and before the back flow or back pressure reaches the valve

I do agreed that the valve will help during the starting phase.

 

[bimr]

Regarding: Air Valve

Anyone who has ever operated a pipeline or water distribution system understands from experience that you should not introduce air into the system. The air will get trapped in the pipeline at numerous high points because the pwater distribution system would never have been constructed with no slope to it. Water distribution systems are not built on level grades, the water pipe profile follows the elevation of the terrain.

I would not recommend draining the entire water distribution system as he is inquiring about.

Regarding valve location:

You can install a valve in the suction line or an enhanced check valve (pump control valve), whatever is easier and cost effective. Note that the poster has not said what size of piping is in the system, but the flow rate is quite large.


Regarding 2 phase flow:

The will enter the poster's entire water distriubtion system downstream of the "vacuum breaker" when the "vacuum breaker" opens and water draings from his "faucets". It is not just drainng at the pump. The water may also drain away from the pump as well if the piping slopes downward from the pumps and air bound the pumps.

How would you propose that it not be 2 phase flow when portions of the water distribution system are drained and refilled?


Don't know what point your are making with your response to 4:

However, here is a quote from your Ventomat article stating that these vacuum breakers and air release valves may be unreliable at low operating pressures:

"It is not possible in practice, to mass produce perfectly spherical balls and generally a working pressure of at least 1 bar is required for a ball float to deform it's resilient seat sufficiently and achieve an acceptable seal."

The point that I have made is at low operating pressues, air release and vacuum breakers will tend to be unreliable.

Regarding will the hydraulic valve still work:

If line pressure is not available, then a secondary source of pressure can be used to hold the valve (check valve, pump control valve, etc) closed. Instrument engineers typically specificy volume tanks for instances where you have no power to shut a valve.

http://www.emersonprocessxperts.com/2009/01/volume_tank_con/

Regarding water hammer and the need to have the check valve close rapidly, here is a quote from the Ventomat article that you reference:

"Check valves are often selected without proper thought to their response UNDER PUMP TRIP CONDITIONS i.e., when a separated column commences to rejoin. The phenomenon of check valve slam occurs due to the fact that very many check valve designs require the reversal of flow to close it; this means that the column
of water is already in motion and stops abruptly as the swing check valve closes, resulting in high transient pressures.
Pressures created in this manner are dependent on the valve design used, the initial pumping velocities and the design head of the system and can be calculated using Joukowski's equation.
IN ORDER TO PREVENT OR MINIMISE THIS PHENOMENON, A QUICK ACTING, SPRING ASSISTED DESIGN SHOULD BE USED THAT WILL REACT IN A VERY LOW MILLI SECOND TIME SPAN AND AT LOW PRESSURES TO CLOSE, WITHOUT SLAMMING BEFORE REVERSAL OF FLOW HAS OCCURRED."

 

[bimr]

Suggest you read thoroughly review the Ventomat article. The article references problems caused by air in the pipeline.

"Air retention results in surge (liquid oscillation) as the water compresses the retained air to a point, whence it acts as a spring, violently oscillating the liquid.

The magnitude of the surge generated is dependent on the water flow velocity (either during initial filling or when separated water columns commences to rejoin) and the size of the entrapped air pocket on closure.

Effect of the strain energy of the surges will be cumulative and concentrated at points of weakness such as reductions in pipe class, fittings which may be of a lower standard than the surrounding pipes, near line valves or tapers and in
branches with closed ends.

Pipes may also fail structurally due to the combined effect of the surge pressures which crack protective pipe linings and the retained air pockets which promotes corrosion.

Retained air causes restrictions which lead to inefficient pipeline operation and increased electrical consumption in pumping schemes as pumps are forced to work at higher heads in order to overcome the restrictions."

 

[Pumpsonly]

The the first part of the Vent-O-mat article discussed the short coming of conventional design air valve and other surge prevention methods. The later part of shows how the CATT design air valve over come or reduce the problem.

I requote:
The fundamental of surge alleviation in the pipe line during a power failure trip is to ensure continue supply of liquid into the pipe line as long as possible to prevent or delay the formation of liquid column separation till the velocity is at it lowest possible to minimize the surge pressure. The surge vessel and fly wheel method typically work on this principle.By closing the valve immediately will cause the liquid separation to happen sooner and at higher run away velocity hence higher impact when the separated liquid columns rejoin.
The check valve are suppose to close when the forward flow ceased
and before the back flow or back pressure reaches the valve causing the valve to slam on its seat.

 

[bimr]

The surge vessel and fly wheel method is an elegant solution that is looking for a problem

If the original poster does not have enough money to ensure a constant water supply and reliable power source, one would doubt that he will spend scarce capital on a more expensive pump.

To summarize,

1. The gooseneck idea is not going to work because

a. The poster will fill up his water distribution system with air which will then be difficult to remove and the air once compressed has a potential to cause water hammer problems in the distribution system.

b. Air release and vacuum breakers are unreliable at low working pressure applications (less than 1 atm pressure) which will be encountered when the pumps and power are off.

c. It is not a good practice to allow air into the water distribution system.

2. His best solution is to install an enhanced check valve where:

a. The check valve will close on power failure to:

1. Prevent siphoning.
2. Help to minimize water hammer potential on pump startup and shutdown.
3. Protect his pump by maintaining water in the pump.

3. A hydropneumatic tank is not practical at a water pumping application where the flows are as high ass 330 lit/sec.

 

[Pumpsonly]

You missed my point of argument.

In the event of a power failure a instant shut down valve at the pump suction or discharge will sure to cause a down surge of pressure in the pipe and hence water column separation and water hammer to follow. Go and read up the book Pumping Station Design by Garr M . Jone. Chaoter 5.7 page 5-27 and chapter 6.7 page 6-11.

A preview copy is available at the below link.

http://books.google.com.my/books?id...ow water column separation in pipeline&f=true

BTW, Cla-Val also supply Air release/ vacuum breaker valve that claimed to alleviate water hammer/ pressure surge in the vent of pump trip

http://www.cla-val.com/pdfs/E-38VBAR.pdf

 

[bimr]

Nobody is advocating an instant shut off valve. A valve was proposed to eliminate siphoning, but it was never proposed to hammer the valve shut.

If you look above, I have also provided a link to a device that is commonly used to slow a valve closing.

However, a quick closing check valve is recommended by most knowledgeable people to prevent the water hammer and backflow in the event of pump failure.

Go and read up the book Pumping Station Design by Garr Jones page 7-7

"Ensure quick closure of the check valve before the flow can reverse to cause a slam..."


"The fast-closing check valve not only prevents reverse flow through the pump, but also provides redundant protection of the pump should the pump control valve fail to close due to loss of pressure or equipment malfunction"

http://www.valmatic.com/pdfs/SurgeControlinPumpingSystems3-17-09.pdf

"Should a power failure occur, a built-in lift-type check valve closes the moment flow stops, preventing reverse flow regardless of solenoid or diaphragm assembly position."

http://www.cla-val.com/pdfs/E-60-73.pdf