What Affects NPSH?

Source Liquid Level –

Changes in the liquid level feeding to the pump

will affect NPSH. A raised liquid level in the tank will increase NPSH

_A

, while a lower than expected liquid level will cause NPSH

_A

issues.

What are the factors governing available NPSH?

• NPSHR: the minimum NPSH given by the pump manufacturer.
• NPSHA (Available): the amount of absolute suction available to suppress boiling at the impeller eye. …
• NPSH3: the net positive suction head available based on a 3% head drop.

How do you increase NPSH?

You can improve the NPSHa by either

increasing the elements that add energy to the liquid (Hs, Hp)

, or by reducing the elements (Hvp, Hf, Hi) that subtract energy from the liquid. Hs is suction static head. It is the elevation of the liquid in the suction vessel referenced above (or below) the pump centerline.

What is the effect of NPSH?

The net positive suction head is the most critical factor in a pumping system. A sufficient NPSH is essential, whether working with centrifugal, rotary, or reciprocating pumps. Marginal or inadequate NPSH will cause cavitation, which is the formation and

rapid collapse of

vapor bubbles in a fluid system.

How can NPSH be avoided?

1. Throttle the pump discharge using a throttling valve or a restriction orifice. This will increase the pump head by reducing the flow rate and operate the pump in a lower NPSHr region.
2. Use an oversized pump.
3. Use an impeller with a larger diameter eye.

Why is cavitation bad?

Cavitation

causes pump performance deterioration, mechanical damage, noise and vibration

which can ultimately lead to pump failure. Vibration is a common symptom of cavitation, and many times the first sign of an issue. Vibration causes problems for many pump components, including the shaft, bearings and seals.

Why is NPSH important?

The purpose of NPSH is

to identify and avoid the operating conditions which lead to vaporisation of the fluid as it enters the pump

– a condition known as flashing. In a centrifugal pump, the fluid’s pressure is at a minimum at the eye of the impeller.

What happens if NPSH is negative?

The NPSH is supplied from the system and is solely a function of the system design on the suction side of the pump. …

If the liquid is below the pump

, this becomes a negative value.

What is the difference between NPSH available and NPSH required?

NPSH can be defined in two parts:

NPSH Available (NPSH

_A

): The absolute pressure at the suction port of the pump. NPSH Required

(NPSH

_R

): The minimum pressure required at the suction port of the pump

to keep the pump from cavitating. NPSH or Net Positive Suction Head is a very important part of a pumping system.

How is NPSH calculated?

NPSH (A)

= (Pressure on the liquid surface) (ha) ± (suction head/lift)

(friction loss) (vapour pressure) (hvp) NPSH (A) = 9.02 + 4-1.2 – 0.25 = 11.57 Mts.

How do you maintain NPSH?

1. Increase the liquid level in the suction vessel.
2. Eliminate any flow restrictions in the suction piping (such as a strainer)
3. Operate at a flow rate less than the pump bep (see figure 3).
4. Install an Inducer, if available.

Does NPSH change with pump speed?

NPSH-Required increases by the 4/3 power of the pump speed

, for a given flow rate and Suction Specific Speed, so assuming that the value of the system NPSH-Available is fixed, lower pump speed can increase the NPSH-Margin, and therefore, pump reliability.

How do you prevent pump cavitation?

1. NPSHa > NPSHr + 3 ft or more safety margin.
2. Lower temperature.
3. Raise liquid level in suction vessel.
4. Change out pump type.
5. Reduce motor RPM.
6. Use impeller inducer.
7. Increase diameter of the eye of impeller.

Does NPSH change with temperature?

However, as you approach and surpass temperatures of 100°F, the vapor pressure begins to

exponentially increase

. This concept is crucial to understand because the temperature of the liquid impacts vapor pressure, which affects the NPSH of your pump.

What is the effect of temperature changes on the NPSH?

The temperature of the Liquid impacts the NPSH req of a pump. As the pumping liquid temperature increases,

the size of the vapor bubbles due to cavitation get smaller

, hence leading to lesser damage.

What is NPSH required?

NPSH Required (NPSHR): The

minimum pressure required at the suction port of the pump to keep the pump from cavitating

. NPSHA is a function of your system and must be calculated, whereas NPSHR is a function of the pump and must be provided by the pump manufacturer.