Suction-lift

Suction lift is negative suction head. It exists when the liquid level in the suction vessel is below the centerline of the pump. The pump must aspirate the liquid up from the suction vessel into the pump and then... 

According to the Standards of the Hydraulic Institute, a suction lift test is performed on the pump and the pressure in the suction vessel is lowered to the point where the pump suffers a 3% loss in total head. This point is called the NPSHr of the pump. Some pump manufacturers perform a similar test by closing a suction valve on a test pump and other manufacturers lower the suction elevation. 

Now let s consider Figure 2-2. This is a pump in suction lift draining an opened tank that s 8 feet below the pump centerline. This pump is installed high on a mountain at 7,000 feet above sea level. The Ha is 26.2 feet. The Hsj is -8.0 feet. The water temperature is 50° F, so the Hvp is 0.411. The Hf is 1 foot and the Hi is 2.0. According to the information ... 

No pump in the world can lift cold water 40 ft from an open well in a suction lift condition because the water would evaporate before it comes into the pump. The rca.son lies in the basic head formula ... 

You can only raise a column of cold water in a pipe a maximum of. S.S.9 ft with a pump in suction lift. Beyond 34 ft, the water will boil or vaporize. This is the reason why submersible pumps and vertical turbine pumps exist. I here is no limit to the distanee you can push a liquid from below, but you can only a.spirate a liquid a maximum of 34 ft from below the pump. 

Lift pumps and pumps in suction lift conditions. 

Be aware that in some cases, you ll have to live with cavitation. Many pumps suffer cavitation for reasons of inadequate design, hor example, when operating only one pump in a parallel system, this pump tends to go into cavitation. Pumps that perform more than one dut through a valve manifold tend to suffer cavitation. Pumps that fill and drain tanks from the bottom tend to suffer cavitation. The last pump drawing on a suction header tends to cavitate. And of course vacuum pumps and pumps in a high suction lift arc candidates for cavitation. 

Noisy Pump. Cavitation Aspirated Air Excessive Suction Lift Not enough NPSHa Bent Shaft Bound Rotor Worn Bearings... 

The total suction lift is defined as above except the level of the liquid is below the centerline of the pump or the head is below atmospheric pressure. Its sign is negative. Total Suction Lift (TSL) = static lift plus friction head losses. 

This applies because the total head for a pump is total discharge head a( + ), minus ( —) the [suction head, a( + )J, or [suction lift, a( —)]. 

For general service the average centrifugal pump should lift about 15 feet of water on its suction side. However, since each process situation is different, it is not sufficient to assume that a particular pump wll perform the needed suction lift. Actually, certain styles or models of a manufacturer s pumps are often specially adapted to high lift conditions. On the other hand it is unnecessary to select a high lift pump when pressure head or flooded suction conditions prevail. Proper evaluation of suction lift conditions cannot be over emphasized. 

The theoretical maximum suction lift at sea level for water (14.7 psi) (2.31 fi/psi) = 34 ft. However, due to flow resistance, this value is never attainable. For safety, 15 feet is considered the practical limit, although some pumps will lift somewhat higher columns of water. WTen sealing a vacuum condition above a pump, or the pump pumps from a vessel, a seal allowance to atmosphere is almost always taken as 34 feet of water. High suction lift causes a reduction in pump capacity, noisy operation due to release of air and vapor bubbles, vibration and erosion, or pitting (cavitation) of the impeller and some parts of the casing. (The extent of the damage depends on the materials of construction.)... 

Figure 3-36A represents a typical manufacturer s performance curve. The values of NPSHr given are the minimum values required at the pump suction. As mentioned, good practice requires that the NPSH available be at least two feet of liquid above these values. It is important to recognize that the NPSHr and Suction Lift Values are for handling water at about 70°F. To use with other liquids it is necessary to convert to the equivalent water suction lift at 70°F and sea level. 

Total Suction Lift (as water at 70°F) = NPSH (calculated for fluid system) — 33 feet. The vapor pressure of water at 70°F is 0.36 psia. 

What is the Suction Lift value to be used with the pump curves of Figure 3-36A, if a gasoline system calculates an NPSH of 15 feet available ... 

Total Suction Lift (as water) = 15 — 33 = —18 feet. Therefore, a pump must be selected which has a lift of at least 18 feet. The pump of Figure 3-36A is satisfactory using an interpolated Suction Lift line between the dotted curves for 16 feet and 21 feet of water. The performance of the pump tvill be satisfactory in the region to the left of the new interpolated 18-foot line. Proper performance should not be expected near the line. 

If the previous system were at sea level, consider the same pump with the same system at an altitude of 6000 feet. Here the barometric pressure is 27.4 feet of water. This is 34 — 27.4 = 6.6 feet less than the sea level installation. The new NPSHa will be 15 ft — 6.6 ft = 8.4 feet available. Referring to the pump curve of Figure 3-36A it is apparent that this pump cannot do greater than 21 feet suction lift as water or 12 feet NPSHr of liquid (fluid). 

Total Suction Lift as water = 8.4 — 33 = —24.6 feet. The pump curves show that 21 feet suction lift of water is all the pump can do, hence the 24.6 feet is too great. A different pump must be used which can handle this high a suction lift. Such a pump may become expensive, and it may be preferable to use a positive displacement pump for this high lift. Normally lifts are not considered reasonable if over 20 feet. 

Note that the equivalent suction lift must be added to the total discharge head for the pump system to obtain the total system head. Keep in mind that the work the pump must accomplish is overcoming the suction losses (-f or —) plus the discharge losses, that is, + discharge loss (all) — (-f if head, or — iflift on suction losses, all). Thus, the suction lift becomes a ( — )( — ) or a ( + ) to obtain the total system head. Keep in mind that a vacuum condition on the suction of a pump never helps the pump, but in effect is a condition that the pump must work to overcome. 

S = Suction lift, negative suction head, ft S l = Worst case suction side static lift, ft (Figure 3-39)... 

Incomplete filling of the cylinders can result in hammering, which produces destructive pressure peaks and shortens the pump life. Filling problems become more important with higher piston velocities. The suction pressure loss through the suction valve and seat is from 5 to 10 psi. Approximately 1.5 psi of pressure is required for each foot of suction lift. Since the maximum available atmospheric pressure is 14.7 psi (sea level), suction pits placed below the pump should be... 

Suction lift, in Hg (referred to 30-in barorheter) Figure 32.28 Effect of dissolved gas on liquid displacement... 

Any high spot in the suction line can become filled with air or vapor, which, if transported into the impeller, will create an effect similar to cavitation, and with the same results. Services that are particularly susceptible to this situation are those where the pump volume contains a significant amount of entrained air or vapor, as well as those operating on a suction lift, where it can also cause the pump to lose its prime. A concentric reducer can cause a similar effect. The suction of a pump should be fitted... 

A pump must have a continuous supply of fluid available to its inlet port before it can supply fluid to the system. As the pump forces fluid through the outlet port, a partial vacuum or low-pressure area is created at the inlet port. When the pressure at the inlet port of the pump is lower than the atmospheric pressure, the atmospheric pressure acting on the fluid in the reservoir must force the fluid into the pump s inlet. This is called a suction lift condition. 

---