Avoid pumping misconceptions

Are "standard conditions" wreaking havoc on your calculations?

By Dirk Willard

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"The pump discharge pressure will change depending on the liquid," said the client's maintenance engineer. For the sake of decorum, I remained quiet and kept a poker face despite knowing he was wrong. For years, I had specified pumps in psi not head to avoid such confrontations. This and other misconceptions seem to run amok in engineering.

First, let's consider the pump head. In my opinion, head, where used, should be in feet of water at 68°F. Those are the conditions the Hydraulic Institute, an association of pump makers and users, chooses when it talks about pumps. The trouble with many pump curves is that they imply water is at 68°F but never state that. This leaves room for mischief.

When you have discharge head (DH) and suction head (SH) in pressure, e.g., psi, the total dynamic head (TDH) is a fixed quantity (TDH = DH – SH). The pressure depends upon the specific gravity (SG) of the liquid versus that of water at 68°F. You might argue that using the density of water at 60°F would be more appropriate if the operating company uses that basis — but, remember, we're ordering a pump! If the head is in ft-liquid, as it is during energy balance calculations, then the pressure, unless converted, isn't known yet. If stated as a result in pressure, the value remains constant.

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