Suction Head of PD Pumps
Suction Head of PD Pumps
(OP)
Hi Folks
Wondering if you can help. We have a small chemical injection PD pump. We are finding that it is impossible to control the pump and we are injecting far too much chemical.
Is it possible that the chemical is going straight through the pump. Is it possible that the suction pressure is too high and is overcoming the pump? Is it possible the velocity in the line is too high?
I am aware that the NPSHa must be greater than the NPSHr but is it possible to have too much NPSH avialble?
Any comments or advice welcomed.
Regards
Ali
Wondering if you can help. We have a small chemical injection PD pump. We are finding that it is impossible to control the pump and we are injecting far too much chemical.
Is it possible that the chemical is going straight through the pump. Is it possible that the suction pressure is too high and is overcoming the pump? Is it possible the velocity in the line is too high?
I am aware that the NPSHa must be greater than the NPSHr but is it possible to have too much NPSH avialble?
Any comments or advice welcomed.
Regards
Ali





RE: Suction Head of PD Pumps
First, the pump manufacturer probably has a recommendation for a minimum back pressure. If the pump does not have enough back pressure to re-seat the check valves, it will not provide accurate delivery. They usually sell back-pressure valves just for this purpose. They are basically pressure regulators that hold back 30 or 40 psi so that the pump has stable back-pressure against it. This improves the accuracy of the pump and avoids the problem that I described above. This is the preferred solution.
In some cooling tower applications, we simply extended the discharge line to an elevation higher than the top of the supply tank so that there can never be higher suction head than discharge head. The discharge is then introduced to an open line or trough that allows the chemical to free-flow down to the cooling tower sump. You should note that it does not work to just run the discharge line up to elevation and then back down. If you do this, it will just siphon and you can still have the same problem. You have to have a vent at the top to break the siphon.
But, of these choices, the back pressure valve is by far superior. It avoids the over addition of chemical and greatly improved the accuracy.
Johnny Pellin
RE: Suction Head of PD Pumps
RE: Suction Head of PD Pumps
If your suction pressure is higher than your discharge pressure, by an amount that exceeds any spring pressure in the valves, the flow will flow straight through the two valves no matter what the pump does. Essentially, you've plumbed to check valves in series and pushed flow in the unchecked direction.
I don't know what chemical you're feeding, but if the chemical isn't too exotic and is relatively clean, magnetic drive gear pumps work fantastically. We use them for water treatment chemicals where I work. They're available in exotic metals (titanium, alloy 20, stainless, etc.).
If your turndown rates are not high, simply vary the pump speed. If they're very high, put a valve in series with the discharge, and a relief valve piped back to suction set at a reasonable pressure. Then just open or close the valve, and the flow meters almost perfectly. Smooth pulseless flow.
RE: Suction Head of PD Pumps
RE: Suction Head of PD Pumps
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