Pump with closed Discharge - Control system

[baj2]

Good Day All,

I have a centrifugal pump that is operated at times with a closed discharge due to the application. In order to ensure that there is a continuous flow at all times through the pump, I want to install a bypass line on the discharge header that would recirculate flow back to the pump suction in the above condition.


I have been told that a Yarway Automatic Control Valve which senses flow can be used to achieve the above. The valve would recirculate the minimum flow required by the pump. Do anyone have experience with this control valve in a similar system?

Thanks

 

[rmw]

You don't give us much information about the application, regarding what is being pumped, discharge pressures, etc.

Without knowing, one would have to state that the Yarway might be just fine, and then again, it might be overkill.

Often, for clear fluids, and especially saturated fluids, OEM's will calculate and furnish a "minimum flow orifice" to be installed in the discharge piping to the return side, so as to always guarantee minimum flow. They will do it based on what the pump is designed to handle. Make sure you pipe it so that it can't be isolated, and thereby fail to protect the pump, and use a "Y" strainer, at a minimum to keep pipe scale, etc., from plugging the orifice.

If your application is something like a high pressure high flow boiler feedwater pump, then something like the yarway is indicated, and there are others that I am sure other contributors will be glad to mention.

If your application is not a clear fluid, I punt.

rmw

 

[baj2]

rmw
rmw

Thanks for your response.
I will try to explain the system better.

The pump is used for gasolene loading of tank wagons. It is operated continously and the discharge is only closed when we need to switch from one wagon to the next, which might lasts between 5-10mins.

The normal pump flow is 1440 GPM.
The minimum flow that is required by the pump = 700 GPM.
The Yarway valve is flow sensitive so it modulates the flow over the full pressure range.

The scenario is such that the pump will be operating at two conditions:-
1) Normal: 1440 GPM
2) Closed Discharge: 0 GPM(Process Flow)

The yarway valve is installed in-line and modulates based on process flow. Hence in condition 2) the valve will be closed in the process side and the bypass fully open recirculating the minimum flow required by the pump.
Question: Wouldn't in this condition the pump operate where the bypass system tells it to?(and not necessarily the min. flow) If so then the bypass system design will need to be considered. In order words how will the bypass system affect the operation of the pump?

Regards
Baj2

 

[rmw]

The bypass system that I proposed will always pass a specific amount of flow, based on the shut off head of the pump, but less as the pump rides out on it's head curve, so it will add to your pumping load. So, therefore, if your minimum flow required is 700 gpm, then the pump would have to pump up to 2100 gpm when it was pumping normally. If I understand your post, it sounds like your Yarway valve, due to its sensitivity, is already doing this, or more.

So, the answer to your question is yes, the bypass system head loss then governs the operation of your pump at shut off conditions. If the bypass system won't pass the flow, then the flow rating of the bypass valve is a mute point.

It also sounds like the current bypass arrangement will protect your pump for partial flow blockages, (kinked hoses, etc.,)

With this fluid, I would be very careful. This is not a pump you want to dead head very long.

rmw

 

[PUMPDESIGNER]

baj2,

There is so very much information missing that I have a hard time commenting.

What are the problems from running at shut-off? We assume disc friction heats the fluid, but will the pump vibrate itself to death quickly (high Ns)?

How you control the by-pass or even whether or not you use a by-pass is determined by the pump characteristics. For instance, low Ns pumps run smoothly at no flow, they only need cooling flow so a small continuous by-pass with no control valve works fine. If pump is quickly damaged by no-flow then you not only need by-pass, but you need redundancy of some type to ensure there can be no problem, preferably a shut-down control with alarm.

Pilot operated pressure relief valve works very well if suitable for your work, high accuracy and reliability, easy service and adjustment.

The Yarway valve I know nothing about, what is it?


PUMPDESIGNER

 

[TD2K]

Yarway ARC valves are good on clean service and gasoline would qualify for this condition.

I would definitely look at them. For this size of pump, I would not use a continuous flow through a restriction orifice, the pumping costs would be significant and you'd have to oversize your pump to handle both the continuous recycle and the loading rate.

What is the head? For loading gasoline into trucks, I wouldn't expect you would need more than 30 to 50 psi DP across the pump.

You could also install a control loop that would recycle gasoline back to the storage tanks when the flow through the pump is less than the recommended minimum. This design needs a flow meter and a control valve which you can compare against the cost of a Yarway ARC valve.

In the two locations I have worked in where they had these types of pumps, the pumps did not have a minimum flow. The pumps were simply automatically started and stopped based on pressure. They were a vertical pump and had given both sites no problems with this type of operation. I can't quantify right now how long they would allow them to run deadheaded before stopping them, I don't believe it was very long but I can likely get some information if you are interested.

 

[baj2]

TD2K

The minimum flow for this pump was given by the manufacturer. I agree that a continuous bypass should not be used. With regards to automatic start and stop that is not an option because of the high frequency of starts/stop per hour. After weighing the cost of a conventional control system and the yarway valve, the latter is cheaper. I would therefore use the Yarway valve as suggested by you.

However, I was quoted a 6" valve(based on hydraulic requirements) to be used on an 8" line (pump discharge). Do you forsee any problem with this arrangement? Additionally, I were told of having a back pressure regulator on the bypass line which will be piped back to the storage tank. Do you think this is required for the system?
I am not to clear as to when this back pressure regulator should be used.
Pump Diff. Pressure = 40 psi at normal operation
Suction Pressure = 30 psi
Vapour Pressure = 7.3 psig

Best Regards
Baj2

 

[TD2K]

The 6" line sounds quite reasonable. The 8" line is sized for your discharge flow of 1440 gpm while the recyle line would be sized for the pump minimum flow of 700 gpm. Pressure drop on the recycle line isn't as much of an issue since if you don't take the pressure drop in line losses, you'll simply take it across the ARC.

It's not clear to me where they are recommended this backpressure controller, downstream of the ARC? I have a feeling you have mislead them perhaps with your vapor pressure of 7.3 psig and they are trying to prevent flashing in their valve. The vapor pressure of gasoline is in that range but in units of Reid vapor pressure, its true vapor pressure is less than atmospheric.

 

[scalleke]

Are they not recommending the backpressure controller to prevent discharge cavitation at the outlet of the recycle line on the Yarway valve?

That sounds like displacing the problem. Since once you will install a backpressure controller after the yarway valve the same problem will be happening after the backpressure controller.

Question I have is how do you close valves when you will be changing from one wagon to the next? Are your valve actuated or are valve closed manually? If they are actuated can you not just install a bypass orriffice and an actuated bypass valve that will open as the other valves are being closed.

Best Regards
Scalleke