Operation of a centrifugal pump under vacuum

First, a bit of theory.

NPSH — “Net Positive Suction Head,” also known as the cavitation margin — is the most important parameter for evaluating a pump’s suction capability. NPSH defines the minimum pressure at the pump inlet required for cavitation-free operation.

There are two NPSH values: NPSHr (“required”) — the required cavitation margin, i.e., the minimum pressure needed at the pump suction nozzle. NPSHr for each pump is determined through factory testing and indicated on graphs and in tables (see Fig. 1). Note that these data are provided for liquid temperature of +20°C.

Fig. 1

Comparison of NPSH values for pumps operating at 1500 and 3000 rpm.

NPSHa (“available”) — the available suction head in the system where the pump is installed. Since pumps operate within a system (heating, water supply, wastewater, alcohol production, food processing, etc.), pump performance depends largely on the piping layout, the system configuration, shut-off valves, automation, and not only on the pump's construction or manufacturer.

NPSHa (system) must always be greater than NPSHr (pump)

NPSHa > NPSHr

This condition must be met for proper, cavitation-free pump operation.

Let us consider an example of pump operation when pumping liquid from a vessel under vacuum. This could be, for example, a chemical reactor or a distillation/fermentation column, from which stillage is being removed.

The basic scheme of such a process is shown in Fig. 2

The engineering design is shown in Fig. 3

Fig. 2

Fig. 3

NPSHa for a system operating under vacuum is calculated using the following formula:

NPSHa = P + Lh − (Vp + Hf)

P — pressure above the liquid surface in a closed vessel (gauge pressure);

since the vessel is under vacuum, we assume P = 0 (absolute vacuum, although in real systems absolute pressure will never be exactly zero)

Lh — maximum static head (flooded suction height);

Vp — vapor pressure of the liquid at the maximum operating temperature;

Hf — friction losses in the suction pipeline at the required pump flow rate;

From this formula, we can conclude that to increase the pump’s suction capability, one must increase the liquid column height (Lh), reduce the vapor pressure (Vp) — since it depends on temperature, cooler liquid is preferable — and reduce friction losses in the pipeline (increase suction pipe diameter, install shut-off valves of larger diameter).

As shown in Fig. 1, it is preferable to use pumps operating at 1500 rpm instead of 3000 rpm.