### Centrifugal Pump Interview Questions 1 contains following questions which probably asked in technical Interview

1. What are the different types of centrifugal pump?
2. What is the basic difference between single stage and multi-stage centrifugal
pump?
3. How many types of centrifugal pump are available based on the suction and discharge arrangement?
4. What is Pump efficiency?
5. What is specific speed, and what is its effect on the pump curve?
6. What are individual efficiencies that affect operation?
7. How do you preserve efficiency?
8. How does curve shape affect efficiency?
9. When is efficiency important?
10. Please discuss how pumping water differs from pumping 40% Propylene Glycol. Does the impeller have to change trim to produce the same flow and head with a more viscous solution?
11. How Does Pump Suction Limit the Flow?
12. Is it true that if centrifugal pump runs in reverse, it will generate zero head?

### 1. What are the different types of centrifugal pump?

Different types of centrifugal pump are:

A. Single stage

B. Multi-stage

### 2. What is the basic difference between single stage and multi-stage centrifugalpump?

The Single stage pump has one impeller and multi-stage pump has two or more impellers in series.

The discharge of one impeller is the suction of the next one and the head developed in all the stages are totaled.

### 3. How many types of centrifugal pump are available based on the suction anddischarge arrangement?

Based on the suction and discharge arrangement, the type of centrifugal pumps available is: -
A. End suction top discharge.

B. Top suction top discharge.

C. Side suction side discharge.

### 4. What is Pump efficiency?

Efficiency of any Equipment means how it can convert one form of energy to another.

If one unit of energy is supplied to a machine and its output, in the same units of measure, is one-half unit, its efficiency is 50%.

Efficiency of a centrifugal pump is the ratio of the water (output) power to the shaft (input) power and is illustrated by the equation below:

Ef = PW / PS
Where:
Ef= efficiency
Pw= the water power
Ps= the shaft power

### 5. What is specific speed, and what is its effect on the pump curve?

Many pump designers reffers specific speed as the most important contributor to centrifugal pump design.

It allows the use of existing design and test data to design similar higher and lower flow pumps because the specific speed of a pump is independent of its size.

It is best to think of it as an index number that can predict certain pump characteristics.

Viewed this way, specific speed can be useful when selecting a pump for a particular application and predicting premature failure due to off best efficiency point (BEP) operation.

### 6. What are individual efficiencies that affect operation?

#### Hydraulic efficiency.

The shape and spacing of the impeller vanes have an effect on overall pump efficiency.

Although the ideal impeller would have an infinite number of vanes, the real world limits us to five to seven for clear water pumps and even fewer for pumps that handle larger solids.

#### Volumetric efficiency.

Whether the volumetric efficiency of a centrifugal pump is a function of the volute or the impeller is debatable (it is probably both), but I will include its effect here.

Volumetric efficiency represents the power lost due to flow leakage through the wearing rings, the vane front clearances of semi-open impellers and the balancing holes in the rear shroud.

#### Mechanical efficiency

The final piece of the pump efficiency puzzle is that of mechanical losses, although some of these losses are not always included in published efficiency curves.

In the case of a frame-mounted pump, these losses are caused by the shaft bearings and the mechanical seal or packing.

For close-coupled pumps, bearing losses are figured into the motor efficiency. Again the rule of thumb follows that of volumetric efficiency, and losses increase as flow and/or specific speed decrease.

#### Combined efficiency

When looking at the overall efficiency of a pump in operation, the efficiency of the driver must be included, and in many instances, that driver will be an electric motor.

### 7. How do you preserve efficiency?

An important part of the volute is the tongue, or cutwater.

Its purpose is to maintain flow into the throat while minimizing recirculation back into the case.

The optimum clearance between the tongue and the impeller periphery is the smallest distance that does not give rise to pressure pulsations during vane tip passing.

A well-designed pump will have a full-size impeller that meets these clearance criteria. When an impeller is trimmed, this distance increases and allows more fluid to recirculate back into the case.

As recirculation increases, hydraulic efficiency decreases.

### 8. How does curve shape affect efficiency?

A typical performance curve is relatively flat at low values of specific speed (Ns) and becomes steeper as Ns increases.

Pump efficiency is lowest at low values of Ns (500 and below) and increases as Ns increases.

It reaches its maximum in the mid-to-high 2,000 range and begins to decrease above 3,000.

However, the decrease above 3,000 is much smaller than it is below 1,000.
Steeper curves usually offer a greater range of control when operated under variable speed control against some fixed elevation or pressure head.

These pumps can be problematic when running in parallel or starting against varying system head conditions.

Flatter curves work fine in across-the-line applications as long as the static or pressure head remains relatively constant.

They also work well in closed-loop (and most open-loop) circulation applications when operated under variable speed control.

### 9. When is efficiency important?

The power required by a pump is directly proportional to both the flow and the head that it produces.

As flow and/or head increase(s) so does the power required. Conversely, power is inversely proportional to hydraulic efficiency.

For the same flow and head, an increase in efficiency reduces the power requirement.

As the cost per kilowatt hour increases, so will the savings due to increased pump efficiency.

What is the effect of the degree of saturation of dissolved gasses on NPSH?
Compare 100 deg F de-aerated water in a tank with a bladder pressured to 10 psig with a tank without a bladder for the same temperature and pressure, with the pressure provided by, say, a nitrogen bottle causing the water to be saturated with nitrogen.

There is definitely an effect.

The dissolved gas changes the molecular interaction of the liquid in which it is dissolved.

Chemical engineers are familiar with this phenomenon via Henry’s Law, and Oswald coefficient, which relates the V/L (void fraction – the freed-up gas volume to liquid volume ratio) as function of saturation pressure and actual pressure of the mixture.

This is not to be confused with the effect of free gas on pump suction performance, and neither it has anything to do, directly, with cavitation (which is caused by vaporization of liquid and subsequent collapse of vapor bubbles).

The dissolved (not free) gas affects the “ability” of a liquid to become vapor when the pressure drops.

In practice, a good example are cooling water tower double-suction pumps, where the incoming water has been so well aerated when passing through the tower - that a significant amount of air stays dissolved, and reduces the NPSHA.

The NPSH margin (NPSHA-NPSHR) for these pumps is not significant to begin with, and with air affecting the NPSHA, the propensity for these pumps to “get in NPSH trouble” is real.

As an estimate, the reduction of NPSHA for these pumps is about 1-3 feet.

In your case, you should be OK if NPSH margin is good. Also, even if some nitrogen dissolved in water, it will probably stay dissolved and will not come out of the solution at the low pressure inlet areas, because of the time delay – it flows through quickly.

In the cooling tower example, the water stays well dispersed in order to get cooled, i.e. the surface area is extremely enlarged, and air can easily get in.

### 10. Please discuss how pumping water differs from pumping 40% Propylene Glycol. Does the impeller have to change trim to produce the same flow and head with a more viscous solution?

Blankin Equipment

Centrifugal pumps work best on relatively “thin” (i.e. low viscosity)fluids.

The fluid velocities inside the passages of centrifugal pumps are generally much higher then in positive displacement pumps – and higher velocities mean more viscous drag, i.e. lower efficiencies.

Typically, centrifugals are not used above 100 cP or so, although there are exceptions.

Hydraulic Institute Standards have a chart to de-rate the pump flow, head and efficiency (which then allows you to calculate horsepower), as a function of viscosity.

Using this chart, a new (de-rated) H-Q and efficiency curves can be constructed.

The impeller diameter is then determined as usual, using the affinity laws.

### 11. How Does Pump Suction Limit the Flow?

True, BEP is what a pump designed for, and it would be best if it operated there.

However, since the actual operating point is an intersection between the pump curve and a system curve - the pump ends up operating all over its curve, because the system curve changes.

Imagine a discharge valve slowly closing - the system curve (which looks like a parabola) will become steeper - and will intersect the pump curve at lower flow. Same for the opposite - if valve is opening - the system curve becomes "shallower", and will intersect the pump curve at higher flow.

Intersection exactly at BEP is purely coincidental - if the discharge valve is set to make the system curve go right thru the BEP point at the pump curve.

Now, what happens if the valve opens wide enough to get the system curve intersect way past the BEP, at high flow?

Keep in mind that a NPSHr curve also looks like a parabola with flow - it rises sharply at higher flow, past BEP. As it does, the NPSHR gets higher and, eventually, exceeds NPSA (available) - thus cavitation begins.

At low flow, cavitation is not a problem, but "other bad things" happen - the low flow becomes insufficient to "fill the impeller eye", and becomes sporadic, pulsing, etc. - causing pump vibrations, and even mechanical damage.

### 12. Is it true that if centrifugal pump runs in reverse, it will generate zero head?

No. As a rule of thumb, a centrifugal pump running in reverse generates approximately half of its rated head.

However, such operation is very inefficient, and motor horsepower would be much higher, as compared with half head operation of a pump running at the correct rotation.