Close-off pressure for low flow bypass valve

[engrrk]

Hi,
While selecting the low flow bypass valve across the chiller to bypass the chilled water at low load conditions, which of the following should be considered as the close-off pressure for the valve selection?

1) The pressure developed by the chilled water pump at its minimum speed? or
2) The pressure developed by the chilled water pump when it is running at its design full load? or
3) The shut-off pressure of the chilled water pump? or
4) The differential pressure across the bypass valve when it is fully open and when the pump is running at its minimum speed? Or
5) The pressure required at the bypass valve to allow it to close, when the chilled water pump starts to ramp up as the building load increases?

Please help, as I haven't arrived at a firm answer even after exploring about this for a full day. Thanks.

 

[urgross]

I think you are looking for two different dP applications, assuming VFD or IGV.

For minimum chiller flow, your pump needs to be sized that min pump flow is higher than min bypass, or the bypass won't work (not enough flow to keep bundle from freezing). The dP across the bypass should be field set to min bypass flow setting, measured from something similar to an Onicon.

I think you will need a separate dP across discharge to correspond to max flow provided by chiller; something needs to control the VFD/IGV.

 

[SAK9]

Close off rating is the maximum differential pressure the valve -actuator combination can overcome for a tight shut off.The low bypass valve opens when the pump is operating close to its minimum speed.So it is opening against the pump differential head at the minimum speed.The valve needs to close once the pump starts to pick up speed in response to demand for chilled water flow from the field.The valve will need to fully close when the pump is somewhere between its minimum and maximum speed.So if you choose maximum speed pump pressure differential as the close off rating you will be safe.You can only know during commissioning against what actual pressure the valve will be closing.

 

[Drazen]

should be no. 6 - pump/system pressure at minimum required chiller flow.

if it's pump with fixed three speeds, you can just set design rpm, controller cannot change it, so once set up at commissioning the speed is not relevant for further discussion.

 

[ChasBean1]

It is for valve differential as noted by SAK9 and should be rated for at least the shutoff head of the pump. You don't know if the pump will some day be replaced with a larger one, in cases of flow issues or expansion. Look at the curve. If the pump is designed for 400 gpm at 80' head, the curve might show the shutoff head (zero flow condition, top left of curve) to be maybe 110'. 110'/2.31 = 48 psi differential across the pump.

So a 50 psi shutoff would be a minimum criteria for this pump, however I would shoot for a higher rating.

 

[urgross]

You pick the pump based on the chiller, not the other way around, unless cost is not a factor.