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Knowledge of vapor pressure is extremely important when selecting pumps and their mechanical seals. Vapor pressure is the pressure absolute at which a liquid, at a given temperature, starts to boil or flash to a gas. Absolute pressure (psia) equals the gauge pressure (psig) plus atmospheric pressure. Water and water containing dissolved air will boil at different temperatures. This is because one is a liquid and the other is a solution. A solution is a liquid with dissolved air or other gases. Solutions have a higher vapor pressure than their parent liquid and boil at a lower temperature. While vapor pressure curves are readily available for liquids, they are not for solutions. Obtaining the correct vapor pressure for a solution often requires actual laboratory testing.
Cavitation can create havoc with pumps and pumping systems in the form of vibration and noise. Bearing failure, shaft breakage, pitting on the impeller, and mechanical seal leakage are some of the problems caused by cavitation. When a liquid boils in the suction line or suction nozzle of a pump, it is said to be “flashing” or “cavitating” (forming cavities of gas in the liquid). This occurs when the pressure acting on the liquid is below the vapor pressure of the liquid. The damage occurs when these cavities or bubbles pass to a higher pressure region of the pump, usually just past the vane tips at the impeller “eye,” and then collapse or “implode.”
Net Positive Suction Head is the difference between suction pressure and vapor pressure. In pump design and application jargon, NPSHA is the net positive suction head available to the pump, and NPSHR is the net positive suction head required by the pump. The NPSHA must be equal to or greater than the NPSHR for a pump to run properly. One way to determine the NPSHA is to measure the suction pressure at the suction nozzle. NPSHR can only be determined during pump testing. To determine it, the test engineer must reduce the NPSHA to the pump at a given capacity until the pump cavitates. At this point the vibration levels on the pump and system rise, and it sounds like gravel is being pumped.
Centrifugal pumping terminology can be confusing. The following section addresses these terms and how they are used: Head is a term used to express pressure in both pump design and system design when analyzing static or dynamic conditions. Pressure in static systems is referred to as static head and in a dynamic system as dynamic head. Total static head is the vertical distance between the surface of the suction source liquid and the surface level of the discharge liquid. Static discharge head is the vertical distance from the centerline of the suction nozzle up to the surface level of the discharge liquid. Static suction head applies when the supply is above the pump. It is the vertical distance from the centerline of the suction nozzle up to the liquid surface of the suction supply. Static suction lift applies when the supply is located below the pump. It is the vertical distance from the centerline of the suction nozzle down to the surface of the suction supply liquid. Velocity, friction, and pressure head are used in conjunction with static heads to define dynamic heads. Velocity head is the energy in a liquid as a result of it traveling at some velocity V. Friction head is the head needed to overcome resistance to liquid flowing in a system. Pressure head is the pressure in feet of liquid in a tank or vessel on the suction or discharge side of a pump. Total dynamic suction head is the static suction head plus the velocity head at the suction flange minus the total friction head in the suction line. Total dynamic discharge head is the static discharge head plus the velocity head at the pump discharge flange plus the total friction head in the discharge system. Total dynamic suction lift is the static suction lift minus the velocity head at the suction flange plus the total friction head in the suction line. Total dynamic head in a system is the total dynamic discharge head minus the total dynamic suction head when the suction supply is above the pump. When the suction supply is below the pump, the total dynamic head is the total dynamic discharge head plus the total dynamic suction lift. Centrifugal pumps are dynamic machines that impart energy to liquids. This energy is imparted by changing the velocity of the liquid as it passes through the impeller. Most of this velocity energy is then converted into pressure energy (total dynamic head) as the liquid passes through the casing or diffuser. A centrifugal pump operating at a given speed and impeller diameter will raise liquid of any specific gravity or weight to a given height. Therefore, we always think in terms of feet of liquid rather than pressure when analyzing centrifugal pumps and their systems.
Positive displacement pumps have a series of working cycles, each of which encloses a certain volume of fluid and moves it mechanically through the pump into the system, regardless of the back pressure on the pump. While the maximum pressure developed is limited only by the mechanical strength of the pump and system and by the driving power available, the effect of that pressure can be controlled by a pressure relief or safety valve. A major advantage of the positive displacement pump is that it can deliver consistent capacities because the output is solely dependent on the basic design of the pump and the speed of its driving mechanism. This means that, if the required flow rate is not moving through the system, the situation can always be corrected by changing one or both of these factors. This is not the case with the centrifugal pump, which can only react to the pressure demand of the system. If the back pressure on a centrifugal pump changes, so will its capacity. This can be disruptive for any process dependent on a specific flow rate, and it can diminish the operational stability, hydraulic efficiency and mechanical reliability of the pump.
The interdependency of the system and the centrifugal pump can be easily explained with the use of the pump performance curve and the system curve. A centrifugal pump performance curve is a well known shape which shows that the pressure the pump can develop is reduced as the capacity increases. Conversely, as the capacity drops, the pressure it can achieve is gradually increased until it reaches a maximum where no liquid can pass through the pump. Since this is usually a relatively low pressure, it is rarely necessary to install a pressure relief or safety valve. When discussing the pressures developed by a centrifugal pump, we use the equivalent linear measurement referred to as “head,” which allows the pump curve to apply equally to liquids of different densities.
The system curve represents the pressures needed at different flow rates to move the product through the system. To simplify a comparison with the centrifugal pump curve, we again use the ‘head’ measurement. The system head consists of three factors: •static head, or the vertical elevation through which the liquid must be lifted •friction head, or the head required to overcome the friction losses in the pipe, the valves and all the fittings and equipment •velocity head, which is the head required to accelerate the flow of liquid through the pump (Velocity head is generally quite small and often ignored.) As the static head does not vary simply because of a change in flow rate, the graph would show a straight line. However, both the friction head and the velocity head will always vary directly with the capacity. The combination of all three creates the systemcurve. When the pump curve is superimposed on the system curve, the point of intersection represents the conditions (H,Q) at which the pump will operate. Pumping conditions change ONLY through an alteration in either the pump curve or the system curve. When considering possible movements in these curves, it should be noted that there are only a few conditions which will cause the pump curve to change its position and shape: • wear of the impeller • change in rotational speed • change in impeller diameter • change in liquid viscosity Since these conditions don’t normally develop quickly, any sudden change in pumping conditions is likely to be a result of a movement in the system curve, which means something in the system has changed. Since there are only three ingredients in a system curve, one of which is minimal, it follows that either the static head or the friction head must have changed for any movement to take place in the system curve. A change in the static head is normally a result of a change in tank level. An increase in friction head can be caused by a wide variety of conditions such as the change in a valve setting or build-up of solids in a strainer. Both sets of events produce the same result: a reduction of flow through the system. If the flow is redirected to a different location (such as in a tank farm), it means that the pump is now operating on an entirely new system which will have a completely different curve. Thus, it is clear that regardless of the rated capacity of the centrifugal pump, it will only provide what the system requires. It is important to understand the conditions under which system changes occur, the acceptability of the new operating point on the pump curve, and the manner in which it can be moved. When the operating conditions of a system fitted with a centrifugal pump change, it is helpful to consider these curves, focus on how the system is controlling the operation of the pump, and then control the system in the appropriate way.
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