Centrifugal Pump

If the oil temperature rises to near its boiling point (or if decreasing pressure as the oil approaches the pump suction lowers its boiling point to near the oil's temperature), part of the oil can vaporize as it enters the pump. The liquid part will still be "thrown" toward the periphery of the pump, but vapor will accumulate in the "eye" of the pump impeller. This accumulated vapor does not pressurize, and so it doesn't push the peripheral liquid into the pump discharge. In other words, the flow stops. This phenomenon is called "cavitation," and it sometimes gives a crackling noise that sounds like gravel rattling through the pump. The noise is from the collapsing of vapor bubbles, which can have enough surface tension to erode away parts of the pump impeller.

This is just a capsule description. You can find out more by searching on the terms "cavitation" and "net positive suction head." There are good Wikipedia and other Web articles about these.

As Tornado said, cavitation could occur as he described it. Many heavy fuel pumps also use the fuel for lubrication and cooling of the pump. Warmer fuel means less lubrication which leads to failure sooner than expected.

We were taught when fuel temp is high, run the pump a little lower to prevent cavitation and to monitor the pump for the sound (it can sound just like gravel). I have seen impellers removed from pumps that went in new and came out looking like they were sandblasted (made me wonder what happened to all that material that went downstream).

Your pump is right in the fuel so that reduces the possibility of cavitation a little, but monitor the pump and when the temp is high lower the pumping rate if any odd noise is observed.

which can have enough surface tension to erode away parts of the pump impeller.

I am sorry to correct but not the surface tension erodes the parts. There is a more complex phenomenon which consists in the IMPLOSION of the vapor bubbles, this generates shock waves. The combination amplitude + speed of sound energy transfer is the reason for the material destruction due to the micro-plastic local successive deformations and thus local fatigue. Implosion is not due to surface tension but to diffusion of vapor in the oil mass and effect of external pressure on the vapor bubble.

Yes, and just to add, the vapour bubbles are created in the low-pressure zone behind the impeller blade (the zone that is effectively trying to suck fluid in), and the implosion occurs as these vapour bubbles move into the higher pressure zones in front of the next impeller blade.

if temperature increases at the suction this will not effect the pump at all but, the main concern is on the fuel. Knowing the condition which is negative pressure on the suction side, the fuel will be most likely to vaporize (see p-v curves of steam, for comparison) And you know, fuel when in vapor state is easy to ignite & cavitation as mentioned by Tornado above. That is why, designers for fuel piping, recommends pumps to be located very near the source or at a minimal negative suction pressure.

When the fuel temperature increase in the tank - i think there is no significant effect to your pump other than there will be a little bit more volume the pump deliver due to the effect of change in density of the fuel. The fuel is getting lighter when heated, this mean more volume per unit mass. Your pump maintains at rated KW therefore, no effect on the pump but the volumetric capacity it delivers increase by a little amount.

Hope you get the point.

Hello Friend, If inlet oil temperature would increase,it decreases viscosity of the oil which decreases pump discharge pressure.If the pump immersed completely in the oil same phenomenon would occur. reverse case will occur if temperature decreases. If temperature increases flow increases and vise versa Ramaprasad Chakraborty. Mech.Engineer,STPS,WB

Good point I forgot to include, but only a significant factor in heavier fuels.

This is not entirely correct. It's a bit more complicated than this, and difficult to give a precise answer without seeing pump curves, viscosity curves, system curves, etc. Just a few points to consider:

- Discharge pressure is a function of discharge head and fluid density. If the increased temperature reduces the fluid density, and if all else remains the same, then the pressure will drop.

- If a temperature increase results in reduced viscosity, then the pump will give increased flow, head and efficiency.

- The flow and head of a centrifugal pump (ie the duty point on the performance curve) is determined by the system into which it is pumping. So, for example, higher temp/lower viscosity could result in lower system head, which means the pump would give more flow and less pressure.

Sir, You may be correct but one thing is that for fuel oil system centrifugal pump never be used because there is a chance of fire catching if ignition temperature achieved due to charning. .Only screw pump is used for fuel oil system.

In this instance the OP has not specified the type of fuel, so it might not be fuel oil.

However, in the case of fuel oil many pump types are used, depending on the application, including vane pumps, gear pumps, progressing cavity pumps and certainly centrifugal pumps.

How then you compare the effect of viscosity to the flow in comparison to fuel density/specific volume?

Yap, very informative answer you got there. I credit GA for you.

Just to add up.

As both density & viscosity is dependent to temperature.

Viscosity accounts the restraining force against the flow while density dictates the total static head of the system. If this would be written in equation.

Total Pump Head = Totat Static Head + Head loss by viscosity + Head loss by friction & bends + dynamic Head (please correct me if I'm wrong)

Valued information. I couldn't stop voting for you & post 11.