NPSHA and NPSHR for Pumps of Alumina Refinery

Hi Friends,

We all are well aware that Alumina production is a  hydro-metallurgical process having over 350 pumps installed for various duty conditions. Majority of the pumps in Alumina refinery are centrifugal pumps for pumping aluminate liquor and slurry with solids like bauxite, hydrate and bauxite residue. Only very few pumps are reciprocating type used in Alumina refinery particularly for pumping fuel oil to calciners and ground bauxite slurry to high pressure digestion circuit. Considering the importance of pumps right from developing the specifications for procurement to successful operation, I thought of exchanging views about pumps in the interest of youngsters in particular. Selection criteria for slurry pumps used in Alumina refinery have already been discussed in our previous post.

In present post, we will discuss about NPSHA and NPSHR for pumps. NPSHA and NPSHR are the abbreviated terminologies respectively stand for net positive suction available and net positive suction required. Now let us understand about NPSHA first. NPSHA figure indicates the adequacy of fluid flooded at the suction of the pump for required volumetric flow of fluid at specific discharge pressure. 

NPSHA is the combined effect of atmospheric pressure, static head of fluid column, fluid temperature and dynamics of the suction piping arrangement to the pump. The following mathematical equation is used to calculate the NPSHA for any centrifugal pump-

NPSHA = H₁ + H₂ - H₃ - H₄ - H₅

Where,
H₁ = Atmospheric pressure at the pump location,

H₂ = Vertical height of fluid column from centerline of pump to the top surface of fluid in tank,

H₃ = Friction losses in suction lines,

H₄ = Velocity head losses in suction of pump,

H₅ = Vapor pressure of fluid at pumping temperature.

In most of the cases, H₂ will be positive where as in case of vertical sump pumps in Alumina refinery, H₂ will be negative because of negative suction to pump .

The NPSHA is always compared with NPSHR which is given by manufacturer of pumps for required speed, flow rate and discharge head. For proper operation of pump, the NPSHA must be more than NPSHR. The technical reasons and the implications there on will be clear with following clarifications.

When fluid enters the impeller of the pump, the pressure drops drastically. In such situation if pressure drops below the vapor pressure of the fluid, then the fluid will start boiling causing formation of vapors. The mixture of vapor with fluid at the suction of pump reduces the capacity of pump due to cavitation. The cavitation also causes erosion to the pump impeller.

Both NPSHA and NPSHR are generally reported in meters and feet in metric system and FPS system respectively.

Hope, I have explained the NPSHA term with clarity in expression. Other technical aspects including associated detailed calculations for pumps has been covered separately in subsequent  post.

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Thanks and regards.

Kunwar Rajendra