Why pump suction condition is so critical for reliability and performance?

Fallow these standard rules to enhance the performance and reliability.

In process plants, the cost of unscheduled maintenance is typically most the most significant cost of pump ownership. In view of this, pump reliability improvements actively pursued by reliability focused users, with the primary focus normally aimed at mechanical aspects.

Not to be overlooked is the fact that further maintenance cost improvements can achieved through proper pump hydraulic selection and satisfy pump suction condition.

It observed that about 70% time suction conditions are not suitable and resulting in high maintenance cost, process breakdowns. In a recent case, we have decided to put a simple guide line paper to understand importance of pump suction conditions. The critical aspects are discussed in this paper to enhance the pump performance.

The most common from a kinetic pump, by far, as the centrifugal pump. These machines uses the dynamic principle of accelerating fluid through centrifugal activity and converting the kinetic energy in to the pressure. One of the claim advantages of centrifugal pump over positive displacement, these pumps ability to operate over a wide range of flow. Since centrifugal pumps operate at intersection of system resistance curve to pump curve, which is known as a best efficiency point. The little change in system curve will shift the operating point over a desired characteristics curve. 

It was observed that over a fix centrifugal pump population, only 30% population is operating close to or at desired operating point at given efficiency. The 70% of population is operating at different point as predicted in the design node. 

An important aspect of pump hydraulic system design is the suction or inlet conditions. A small gap can result in vortices, cavitation, loss of prime, and mechanical failure. Actually pump do not force liquid force liquids through inlet or suction piping, but rather create a high lower pressure at pump suction which in turns induced the liquid fluid to enter. Hence, any small design gap will operate the pump away from BEP, which results in low liquid flow. In long run, the small issue will result in pump damage or its part apart from the process disturbance.

Any pump suction consideration should be taken care for two things 1) suction source 2) suction piping. The importance must be given to both in order to properly design an efficient system. There are many factors that affects the operation of pump. In this article, we will share the suction conditions, where small gap has made pump not to work at all. Even some of cases the pump is replaced with new pump but still problem was as it is. In any pump, the important and critical factors are total head, speed, liquid properties, and pump arrangement. Excessive suction lift, shallow inlet submergence, or insufficient NPSHA, all spell a serious trouble from vibration, cavitation, lowered capacity, and reduced efficiency. 

The following factors determine the Allowable Operating Region, with the degree of importance dependent on the pump type, specific design, and suction energy level: 

1. Is sufficient NPSH available?

The simple understanding, every impeller requires a minimum amount of pressure energy in the liquid being supplied in order to perform without cavitation difficulties. This pressure energy known as net positive suction head required (NPSHR). To avoid cavitation, the NPSH available from the system must be greater than the NPSH required by the pump. In the simple system as shown in figure-2. The NPSH available can be calculated as follows. 

NPSHA = Ha + Hs - Hvp – Hf

Where Hs = Static head or vertical distance, Ha = Head on the surface Hvp = Vapor pressure of the liquid Hr =suction side friction losses (Ha is in minus in case of suction lift)

NPSH is so essential to trouble free pump operation is always reduced by losses in suction piping. An economic balance must be obtained between pump size and speed, required NPSH, pipe size, and suction velocity. Suppose suction source is tank, such as deaerator in power plant, it is quite expensive to elevate the tank. Therefore, the NPSH available is low. The pipe size from the tank to suction should be large for low velocity and the length should be short for minimum losses. A pit pump should be as close to the suction source as possible. When the NPSH required indicates that the suction lift is possible, the most advantageous solution is to reduce the suction losses by increasing the pipe size.

In general engineering practice holds that suction side fluid velocity should be limited to 5 to 8 ft/sec or alternatively suction side pipe pressure loss should be limited to 2 to 3 ft of the pump liquid. A margin of 2 feet or 1 meter is sufficient to kept between NPSHR and NPSHA,

2. Minimize suction line pressure loss 

When a pump is taking suction from the tank, whether it is flooded suction or suction lift, it should be located as close as possible. It is one of important element which is contributing in NPSH. If the pump is away from the suction source, it is ensuring that correct piping practices can be followed to get optimum result. Improper design of suction piping can cause hydraulic disturbance that manifest in chronic eventually destructive random frequency vibrations. Its lead to enough pumps problem such as bearing failure, seal failure, excessive erosion, excessive corrosion, excessive noise, excessive vibrations. 

The direct elbow to the suction eye is always avoided, if can be avoided than use large elbow 1.5D and use in connection with centric connection as shown in figure -3. The commercially available elbow in two type, one short radius and other is long radius. It is always beneficial to use long radius elbow to minimize the turbulence at the suction. The ideal suction pipe approach to the pump impellers eye is straight pipe. The absolute minimum length of full-size straight pipe should be 10D. This is illustrated in figure 4. 

Pump inlet piping should be designed and install to provide smooth and orderly flow to the pump. The pump suction bell reduces the contraction entrance frictional losses by “smoothing” the flow by elimination of sharp inlet edges. Entrance fluid fictional losses can be reduced by upto 80% depending upon the size of the rounding. The suction bell also reduces the approaching the mean fluid velocity, it has been discussing that minimum submergence depth varies directly to the velocity as its relates to vortex formation.

The check valve is provided to maintain pipe liquid filled condition; it is advisable that provided check valve only is deemed necessary.

A separate and dedicated suction line should be used in situation where multiple pumps are taking suction from common header. Figure-7 shows a plan view of the wrong and correct manner to make header connections. The minimum distance between the connections should be 3D and that Y branches oriented in the direction of flow should be used in lieu of a right angle connections. Figure -6 is showing the correct way to accommodate pipe size changes in pump suction from header to suction take offline.

3. Minimize phase change and use baffle to separate the vortex effect in suction lift 

The three phases of matter are liquid, solid, and gas. The exitance of more than one phases at suction will create problem for the pump operation. Even a small amount of entrained air or gas reduce pump head, capacity, and efficiency. The source of two phases are agitation, vertexing, and vaporization at elevated temperature. Water have also dissolved gases under certain condition, and these come out from the solution in particular pressure.

Need to look on the following aspects, return pipelines to the pits or tanks that allow the liquid free fall, will allow the turbulence at suction and the bubbles entrainment is increased in such cases. Hence, any return liquid line to the pump suction properly dipped and separated by the baffle plate. This will ensure the minimum impact on the pump suction (Figure-8 & 9). 

When the suction line is exceptionally long, or pump is subjected to a large static suction lift, the suction pipe is more susceptible to air leak. Piping that employs non-positive connection type joints e.g. bell, spigot joints, or other pressure tight connections are the only recommended means of joining segment of long or high suction lift condition. 

4. Minimum Submergence in suction lift case 

Submergence is commonly overlooked pump suction side that can create mechanical and hydraulic problems. Insufficient submergence can lead to vibrations, a broken shaft, shorten mechanical seal life, premature bearing failure, lower flow rates, and diminished the discharge head for the pump. In the worst case, these issues will cause surging and stall (air block) the pump.

A common misconception is that inadequate submergence issues are found only on vertical and/very large pumps. This issue, however, can and will occur on small/and horizontal pumps.

When a centrifugal pump operates at given flow rate (Q), there is corresponding fluid velocity in the suction line (V), we can calculate the velocity easily by equation 1. The velocity of the fluid is important value to know because it will determine the correct submergence required to prevent the formation of vortices. 

Figure-10 shows two separate cases, both are for flow rate of 300 gpm, but the suction pipe diameter is 4 inch in one case and 6 inch in other case. Using equation -1 the suction velocity for 4-inch pipes yields to 7.7 ft/s and for 6 inch pipe is 3.4 ft/s. It will clearly be visible for case -1, the submergence is 4.5 feet and for case-2, it is just below the half 2 feet. This is a difference of 2.5 feet.

Do not confuse submergence with net positive suction head (NPSH) available. It is recommended that calculate both value for worst case condition. You can have adequate NPSH and still not have proper submergence and vice a versa. 

It is important to remember, the critical submergence must be greater than simple submergence to prevent vortex. The suction pipe velocities should be kept between 5 to 8 feet/sec. As a general guideline, the recommended suction inlet pipe size can be calculated by equation -2.

Air pockets or high spot in a pump suction line invariably cause trouble. Piping must be laid out so it provides continual rise or at least perfect horizontal run without high spot from the source of supply to the pump. For the same reason, an eccentric reducer instead of straight taper should be used in horizontal suction line. Another way to remove trapped vapor in a suction line because of a high point is to vent the sector vapor back to the vapor space in the supply vessel. If an air pocket is left in the suction line pipe when the pump is primed, it will often pump properly for a time than it lose its prime or its capacity greatly reduced. The small pocket of air under a partial vacuum condition will greatly expand and reduce the effective flow cross section of the pipe, thus starving the pump. This case much possible where ever foot valve are not easy to install because of liquid characteristics.

5. Pump intake Arrangements

Pump suction or intake structure can be categorize as being for clear liquid or solid bearing liquids, intake are further classified into rectangular, formed, circular, and trench type, as well as suction tanks and cans.

These structures are covered in detail in American National Standard for pump intake design, ANSI/HI 9.8 -1998

6. Case Study: Bleach Pump suction lift & cavitation problem 

In a recent case, the one of bleach slurry pump is not taking the load even after prime, it is losing suction and cavitating. Even the maintenance team has changed the pump with new pump and the result is same. Pump is not transferring the fluid. This is causing production loss as both pumps are not working. The arrangement of the pump is shown in figure below.

During review of suction condition, there were two critical gaps were observed. The first gap is submergence of the pump, the NPSH is adequate and it is observed that for 3” suction line velocity is coming close to 10ft/s. From the figure 10, the required submergence is >6 feet, where as the pit was close to 6 feet.  

Second, the suction pipe does not have bell type entrance and it is not provided with baffle to avoid the turbulence in pump suction. The agitator is operating close to 60 rpm and providing enough turbulence and bubbles to the suction of the pump.

Two action point was suggested, one is change in the minimum submergence requirment by changing the line size from 3” to 4” and reduce the rpm to desired level, provide baffle to separate out the suction pipe from turbulence.

It is also observed that the suction pipe has more than 3 short elbow in place of long elbow, which is not best suction pipe arrangement in any suction lift condition.

However, after reducing the agitator speed, pump has taken the load. It is still operating in very close range of cavitation and small increase in pressure drop will lead poor performance. The pump suction condition change modification is taken. It is to operate the pump in best AOR.

AOR: Allowable Operating Range

Summary

Pumps problem often begin on the suction side. Hence, while working on the pump selection, the suction side condition will be calculated with extra care. This is possible during the design phase. 

It is a good practice to maintain pumps history card and easy available with number of issues occurred. Ensure to take a photograph, if any failure has occurred. Many of time, the depth analysis of pump problems are not carried out and it is providing repetitive failure with same frequency. In such cases, the team has to refer the root cause and try to come out with permanent solution.

When purchasing a new pump or replacement, for a sensitive suction services, go beyond the cost and satisfy following questions.

1. Were the pump’s calibration tests conducted in conformance with Hydraullic institute standards?
2. Can the vendor provide performance specific data to your anticipated suction requirment?
3. Can vendor furnish a typical pump impeller and casing set to examine?
4. The valve and fittings considered at the pump suction are adequate?
5. Suction pipe should be designed and install to minimize the entrap air, this consist of positive sloped lines?

Table- Some of ANSI recommended value

LITERATURE CITED 

1 Practical consideration in pump suction arrangement, Randall W. whitesides, CPE,PE

2 Handbook of Centrifugal Pump, Mcgraw-Hill, W.C.Kurtzch, Paul cooler, 3 edition