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Understanding Pumps Terminology

As a process engineer, I sometimes dealing with pumps. I usually feel confuse when I read pump specification or graphs offered by vendors. Here in this post, I want to share you some knowledge about pumps terminology that you may hear before what don’t really understand the meaning.

I create mind map for several most-often-used pump terminology that come to my mind. I’m sorry if the arrangement is a bit messy :D.

pump-terminology

FLOW

  • Best Efficiency Point (BEP). It is flowrate at which a pump achieve its best efficiency at rated impeller diameter (definition based on API 610). By operating at highest efficiency, running cost of pump will be the lowest.
  • Rated Operating Region. It is region at wh5ch the vendor certifies that pump performance is within the tolerances stated in API 610. It is used as basis to buy the pump. In other words, the vendor shall guarantee that the pump can be operated in its rated flow, however in actual work, the pump will not operated at its rated flow. API 610 suggests rated flow shall be within 80% to 110% of best efficiency point
  • Preferred Operating Region. It is portion of pump’s hydraulic coverage over which pump’s vibration is within base limit of API 610.
  • Allowable Operating Region. This is region of pump’s hydraulic coverage over which pump is allowed to operate in whilst conforming to predefined API 610 vibration limits.
Typical Curve of Centrifugal Pump
Typical Curve of Centrifugal Pump

MINIMUM FLOW

We heard much about minimum flow, but what actually does it mean?

Minimum flow is the lowest continuous flow the pump is permitted to operate without reference to a specific vibration limit. It is usually recommended by vendor.

API 610 differentiate minimum flow into:

  • Minimum continuous stable flow. It is the lowest flow at which pump can operate for which specified vibration limitations will not be exceeded. It is used to prevent cavitation and vibration
  • Minimum continuous thermal flow. It is the lowest flow for which maximum permissible pump temperature rise is anticipated. It is used to explain about temperature rise of liquid.
Pump Phenomena vs Flow
Pump Phenomena vs Flow

HEAD

I sometimes confuse when it comes to “head”. I usually ask myself, “Does it refer to discharge head or differential head?”. After I browsed, I find that head is mostly used as an abbreviation of differential head. In data sheet, I think it will be better to use specific term, such as suction static head and suction discharge head. 

NET POSITIVE SUCTION HEAD (NPSH)

NPSH is simply defined as the difference between suction head and liquid vapor head. In other words, it is reduction of pressure as the fluid enters the eye of the impeller. If the drop in pressure falls below the vapor pressure, fluid phase changes from liquid to vapor. NPSH shall be calculated and checked to see if the pump will operate appropriately. Low pressure at suction pump will cause fluid to boiling. In the end, it will lead to reduced efficiency, cavitation, and pump damage.

Cavitation phenomena
Cavitation phenomena

NPSH can be divided into:

  • NPSH Available (NPSHA). It is absolute pressure at the suction port of pump. NPSHA is function of system, therefore is must be calculated.
  • NPSH Required (NPSHR). It is minimum pressure required at the suction port of the pump to keep the pump from cavitating. NPSHR is function of pump and must be provided by pump vendor. NPSHA must be greated than NPSHR.

SHUTOFF PRESSURE

Shutoff pressure is maximum pressure a pump will develop under zero-flow condition, which reflects fully blocked outlet.

THROTTLING

Throttling is introducing a pressure drop in a fluid circuit. It is commonly performed by using globe valve. It converts head into heat, which increases liquid temperature.

References:

  1. API 610 – pump selection & curve evaluation guide
  2. Purpose of minimum flow
  3. Technical definitions for pumps – part three
  4. Water and air flow meter
  5. Understanding net positive suction head
  6. Understanding the basics of centrifugal pump operation

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