Basic questions on Pumps

Question 1: Does excessive amount of air at the pump suction cause cavitation?

Answer: No. Air has nothing to do with it. Cavitation is caused by the collapsing (imploding) vapor (not air) bubbles. These bubbles are simply a vaporized liquid in the region where static pressure dropped below vapor pressure. Air causes other problems, such as air locking, and even a very small amount of it causes significant loss of performance (head drops), but this is a different subject.

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Question 2: Can a gear pump "lift" liquid? What is dry lift?

Answer: Yes. Gear pumps have good lift characteristics, in the range of 5-20 feet, depending on the particular design. Lift characteristics of a gear pump improves significantly if even a minute amount of liquid is initially allowed to "wet" the internals, which is often the case if a pump was tested at the factory, and some residual liquid remains, or intentionally pre-lubed at the site. This minute amount of liquid acts as a capillary barrier in the clearances, preventing air from escaping back to suction during startup at lift. With no pre-lube, gear pump will still lift, but not as good.

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Question 3: What determines number of stages of the progressing cavity pumps?

Answer: Number of stages depends on several factors, with the main one is total differential pressure. Typically, a stage is added for each 75-100 psi. For example, a 300 psi differential would require 4-5 stages. Manufacturers catalog provides different curves for different stage number designs.

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Question 4: Is it true that if centrifugal pump runs in reverse, it will generate zero head?

Answer: 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.

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Question 5: "How Does Pump Suction Limit the Flow?"

Answer: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.

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Question 6: 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

Answer: 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.

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Question 7: 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.

Answer: 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.

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Question 8: Explain about

1)backward curved blades

2)forward curved blades

3)comparison of above

4)which is more advantageous and why?

Answer: The shape of the blades depends on the details of the hydraulic design. A centrifugal pump operates on a principle of imparting an angular momentum to a fluid, i.e. literally the fluid must change direction as it passes through the impeller blade cascade. In other words, there is an exchange of energy, as a mechanical torque is transmitted from the motor shaft to, ultimately, the hydraulic energy, which manifests itself in building a pump pressure. Hydraulic designers refer to this as “velocity triangles”: one at the impeller blade inlet, and another at the exit. A velocity triangle has peripheral velocity (U), absolute velocity (V), and relative velocity (W), as shown below, with impeller rotating clockwise:

Figure Q8-1 “Backward-curved” blades, - most pump impeller designs

The ability of building up pressure depends directly on product U x V_theta , which means that for higher pressure the impeller OD must be larger, or the pump should rotate faster – these make velocity “U” vector longer. The relative velocity vector (W), including its magnitude and direction, must be such that the velocity triangle closes-up to produce the desired pressure and flow. For most centrifugal pumps, this relative velocity vector ends up (anti-intuitively) “backward”, against the direction of the velocity “U”. The angle between the vectors U and W is called a relative flow angle “beta”, and the blade angle is set approximately equal to that. This angle “beta” ranges between 10^O to 35^o , for most single-stage centrifugal pumps, but, at higher values of Specific Speed (NS), such as turbine pumps, it can be as high as 40^o to 50^o

Some machinery has significant space limitations, such as car hydraulic transmissions. There, it is not possible to “beef-up” velocity U by large impeller OD, and the only option is to curve the blades “forward”, to still create a large velocity V_theta , and thus build up the same pressure as was in Fig. Q8-1 above:

Figure Q8-2 Special “forward-leaning” blades, such as in a pump of a hydraulic transmission

While this allows significant size reduction, the downside is low efficiency, because the absolute flow velocity becomes too large, and would result in increased hydraulic losses for a “normal” pump. In hydraulic transmission, however, there is a pressure recovery turbine, which sits immediately after the pump. The blades of the turbine wheel are also curved in a “funny fashion”, to accommodate and match the exit velocity triangle of a pump. Thus, the turbine “picks up and recovers” the velocities produced by the pump.

As most pump impellers discharge directly into a volute, or a diffuser, without having a special recovery turbine wheel following the pump impeller, the majority of the designs have backward-leaning blades.

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