Boiling Fluids

Boiling Fluids - Recommended Suction Flow Velocity Recommended flow velocity for the pump suction side Capacity problem, cavitation and high power consumption in a pump, is often the result of the conditions on the suction side. In general - a rule of thumb - is to keep the suction fluid flow speed below the following values:

Boiling liquids - due to the cavitation problem - may be extremely difficult to pump. If the pressure at any point within the pump falls below the vapor pressure of the liquid being pumped, vaporization or cavitation will occur. For example, water at 100oF (38oC) boils or vaporizes if exposed to a vacuum of 28 in Hg. The problem with cavitation increases with higher temperature. Condensate Pumping High temperatures and danger of impeller cavitation is the major challenge of condensate pumping Often its necessary to pump generated condensate from heat exchangers and other consumers widely distributed in the plant, back to the condensate receiver in the boiler house. A special challenge with hot condensate, which is often close to 212oF (100oC), is cavitation of the pump and the pump impeller. Centrifugal pumps generates lower pressure behind the wheels, and the hot condensate temporarily evaporate and expand on the back side of the vanes - before it implodes and condensate. Over time this will erode and destroy the pump impeller. To avoid the problem there are two alternative solutions: 1. Add pressure to the suction side of the pump 2. Use a pressure powered pump instead of a centrifugal pump Add Pressure to the Suction Side of the Pump If the absolute pressure exceeds the vapor pressure at the actual temperature of the fluid entering the pump, then the Net Positive Suction Head (NPSH) is positive and its theoretical possible to avoid cavitation. A NPSH above the manufacturers specification is important to avoid the water start boiling behind the impeller. The NPSH can be expressed as:

NSPH = 144 / ρ(pa - pvp) + hs - hf

(1)

where ρ = density of water at the appropriate temperature (lb/ft3) pa = absolute pressure in the condensate receiver supplying the condensate pump. This is the same as atmospheric pressure if the receiver is vented (psi) pvp = absolute pressure of condensate at the liquid temperature (psi) hs = total suction head in feet. Positive for a head above the pump and negative for a lift to the pump. hf = friction loss in the suction piping According to (1) the NPSH can be increased by -

increasing the difference in the pressure in the receiver and the condensate pressure, and/or extend the static difference hs by lifting the receiver or lowering the pump, and/or increasing the piping dimensions for minimizing the friction loss hf in the suction pipe

If it's not possible to increase the suction pipe and lowering the pump regarding the receiver, it's possible to reduce the absolute pressure of the condensate Pvp, by reducing the condensate temperature with a cooling exchanger on the suction pipe. Use a Pressure Powered Pump A pressure powered pump use steam or air pressure to push the condensate from the receiver back to the boiler room. In principle its a simple mechanical construction working in a cycle where a receiver is filled with condensate before the condensate is pushed out and back to the boiler room. The pump don't need external power, it can use the available steam (or pressurized air), and there is no danger for cavitation. Other Liquids as LPG The pumping of other boiling liquids - as LPG (-43oC in normal atmospheric pressure) - offers the same challenges to the manufactures and users. LPG is stored at exactly its boiling point (at the actual pressure in the tank) and any increase of temperature, as well as any decrease in pressure, will cause the product to boil and form vapor. In many installations, the suction friction head is equal or larger than the static suction head, making the available NPSH a negative value. The pressure drop due to the flow restrictions in the inlet piping system, e.g., excess flow valve, control valves, fittings, strainer, etc., will induce the LPG vapor formation at the pumps suction port.

Cavitation - an Introduction Cavitation may occur in fluid flow systems where local static pressure is below the vapor pressure Cavitation is a common problem in pumps and control valves - causing serious wear and tear and damage. Under the wrong conditions, cavitation reduces the component life time dramatically.

What is Cavitation? Cavitation may occur when the local static pressure in a fluid reach a level below the vapor pressure of the liquid at the actual temperature. According to the Bernoulli Equation this may happen when the fluid accelerates in a control valve or around a pump impeller. The vaporization itself does not cause the damage - the damage happens when the vapor almost immediately collapses after evaporation when the velocity is decreased and pressure increased.

Avoiding Cavitation Cavitation can in general be avoided by

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increasing the distance between the actual local static pressure in the fluid - and the vapor pressure of the fluid at the actual temperature

This can be done by:

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reengineering components initiating high speed velocities and low static pressures increasing the total or local static pressure in the system reducing the temperature of the fluid

Reengineering of Components Initiating High Speed Velocity and Low Static Pressure Cavitation and damage can be avoided by using special components designed for the actual rough conditions.

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conditions as huge pressure drops can - with limitations - be handled by Multi Stage Control Valves challenging pumping conditions - with fluid temperatures close to the vaporization temperature - can be handled with special pumps - working after other principles than centrifugal pumps

Increasing the Total or Local Pressure in the System By increasing the total or local pressure in the system the distance between the static pressure and the vaporization pressure is increased and vaporization and cavitation can be avoided. The ratio between static pressure and the vaporization pressure - an indication of the possibility of vaporization, is often expressed by the Cavitation Number. Unfortunately it may not always be possible to increase total static pressure due to systems classifications or other limitations. Local static pressure in components may be increased by lowering the component in the system. Control valves and pumps should in general be positioned in the lowest part of the systems to maximize static head. This is common solution for boiler feeding pumps receiving hot condensate (water close to 100 oC) from condensate receivers.

Reducing the Temperature of the Fluid The vaporization pressure depends of fluid temperature. Vapor pressure of Water, our most common fluid, is indicated below:

Note! - the possibility of evaporation and cavitation increases dramatically with the water temperature. Cavitation can also be avoided by locating components to the coldest part of a system. It is common to locate pumps in heating systems in the "cold" return lines. This is the same for control valves. If it is possible control valves should be located on the cold sides of heat exchangers.