The pumps minimum requirements NPSH

The last component of the pump performance curve is the curve of the minimum requirements, or NPSH. Actually, the reading on the pump curve is the NPSHr, the Net Positive Suction Head required by the pump. There is a complete discussion on NPSHr and NPSHa, and the result of not respecting or understanding them in Chapters 3 and 4. Basically, the NPSHr curve, beginning at 0 How, is mostly flat or modestly rising until it crosses through the BFP zone. As the NPSHr curve crosses through the BFP of...

Determining the Hp

The Hp also can exist with the pump running or off. We can represent this value with an O or oval on the vertical line of the below graph. The Hp is added to and stacked on top of the Hs. Let's say that our system is pumping cold water and requires 50 ft of elevation change and 10 psi of pressure change across the system. Now, our pump not only has to lift the liquid 50 ft, but it must also conquer 23 ft of Hp. Remember that 10 psi is 23.1 ft of Hp Here is the system curve showing Hs and Hp...

339 ft

You can only raise a column of cold water in a pipe a maximum of 33.9 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. There is no limit to the distance you can push a liquid from below, but you can only aspirate a liquid a maximum of 34 ft from below the pump. Question If you put a straw into a glass of milk and suck on the straw, are you really Reply If you could really suck on the milk,...

The energy BHp curve

Next, let's consider the energy curve, the brake horsepower (BHp), required by the pump. This curve is probably the easiest to interpret because it is practically a straight line. Consider the following the pump consumes a certain quantity of energy just to maintain shut-off head. Then, as flow begins and increases, the horsepower consumption normally increases. (On certain specific duty pumps, the BHp may remain mostly flat or even fall with an increase in flow.) The BHp curve is normally seen...

Oring The elastomer failure

About half of all pumps in the shop today were pulled out of service because they were leaking or wouldn't hold pressure or pump. This is most likely a leaking o-ring. The o-ring is the rubber component of most mechanical seals. The o-ring controls the temperature, pressure, and chemical resistance of the mechanical seal (Figure 14-1). The difference between a mechanical seal in a pump in alcohol service and a pump in steam service is the o-ring. It is not the stainless steel, or the ceramic...

Hq

The matrix of the pump curve graph is the same as the mathematical 'x-y' graph. On the horizontal line, the flow is shown normally in gallons per minute or cubic meters per second. The vertical line shows the head in feet or meters. See Figure 7-1. By definition, the pump is a machine designed to add energy to a liquid with the purpose of elevating it or moving it through a pipe. The pump can elevate a liquid in a vertical tube up to a point where the weight of the liquid and gravity will...

NPSHa Ha Hs Hvp Hf Hi

Where Ha Atmospheric head (14.7 psi x 2.31) 33.9 ft. at sea level. See Properties of Water 1 in this chapter that considers atmospheric pressure at different elevations above sea level. Hs Static head in feet (positive or negative) of the fluid level in the suction vessel to the pump centerline. Hvp the Vapor head of the fluid expressed in feet. It is a function of the temperature of the liquid. Sec Properties of Water II in this chapter. Hf Friction head or friction losses expressed in feet in...

The facetoface dual seal

This facc-to-facc double seal is quite versatile (Figure 13-21). It is recommended in a wide range of applications depending on the piping arrangement of its support system. With its support system pressurized above the seal chamber pressure, this double seal functions well with toxic and dangerous liquids (like the back-to-back dual seal). If the support system is pressurized at J 2 the seal chamber pressure, this seal can handle higher pressures where a single seal would fail. If the support...

Introduction

Along with the sounds, evidence and signs of cavitation, there is a broad range of other information and signals available to the maintenance mechanic. Almost all mechanics have seen the gouge and scratch marks, and signs of heat on the pump when disassembled in the shop. Sadly, most mechanics are never trained to interpret these marks. This brings us to failure analysis of the pump, or performing an autopsy on a broken pump. You must stop throwing away used and worn pump parts, or sending them...

Three tips

First, one pump running in a parallel system tends to suffer from cavitation because operation to the right of the BF.P indicates that the NPSHr of the pump rises drastically. To survive this condition, you should use dual mechanical seals on these pumps. Dual or double mechanical seals can withstand cavitation better than a single seal. There is a discussion on this in the mechanical seal chapter of this book. Many engineers perceive that parallel pumps are problematic because they appear to...

Bearing maintenance

A lubricant, either oil or grease, should always be present in the bearings in small quantities. If not, the life of the bearing will be compromised by damage to the bearing surfaces. This damage can be avoided with proper cleaning and re-lubrication. The intervals for cleaning and re-lubing the bearings are generally long periods. It's easy to see when a bearing needs oil. Check the oil site level indicator. It's different with grease. It's impossible to determine when a bearing needs more...

Centrifugal pumps

Centrifugal pumps perform the same function as PD pumps, but they do it differently. These pumps generate pressure by accelerating, and then decelerating the movement of the fluid through the pump. The flow, or gallons per minute, must be available to the pump's suction no . .le. This flow, or energy, in centrifugal pumps is called NPSH or Net Positive Suction Head (discussed in Chapter 2). These pumps, like their PD sisters, cannot generate flow. No pump in the world can turn three gallons per...

Pump packing

Many industrial pumps around the world still use ring packing as the shaft seal. These packings require continual maintenance. They need to be tightened and adjusted and it is necessary to change them frequently. Packings wear and damage the shaft or sleeve of the pump, which necessitates their frequent replacement. The leakage from the packing gland corrodes the pump, base plate and foundation. The drips can enter into the bearings and contaminate the bearing lubricant. This sacrifices the...

Causes of premature seal failure

There are numerous reasons why seals fail prematurely. The origin of the failure can reside in the pumping system, in the pump operation, or the maintenance shop, the storeroom, or even before the seal arrived into the plant. The first sign of failure normally is liquid on the floor. It's possible that a failure occurred to the seal at the manufacturer. Nowadays, the companies that we call 'Manufacturers' or 'Factories' are actually 'Parts Assemblers'. It's likely that no mechanical seal...

The single balanced external mechanical seal

Outside balanced seals permit sealing pressures up to about 150 psig. They offer the other benefits of outside seals, which make them popular in the food processing industry (Figure 13-7). In the food production and OTC (Over the Counter) Drug industries, like milk, soups, cough syrup, and juices, outside balanced seals are quite popular. Their design permits easy cleaning of the equipment without pump disassembly. These seals are prominent in the chemical processing industry because all metal...

Pumps in parallel

The system is designed so that two equal pumps are operating together side by side. The system can support the production of both pumps. If the needs of production are reduced, this system can operate with only one pump, simply by removing one pump from service (Figure 8-25). The curves of pumps 'A' and 'B' individually, and 'A and B' in parallel arc seen in Figure 8-26. Because the system is designed for both pumps running together in parallel, the system curve appears as shown in Figure 8-27....

Rotary action

Reciprocating action in engines and pumps was again converted back again into rotary action at the beginning of the last century. First, the rotary turbine was perfected. Shortly afterward, the internal combustion engine appeared. In the marine industry, the propulsion paddlewheels evolved into propellers. Ship design was greatly simplified with a direct drive shaft from the motor to the propellers. The weight and spacc of the complicated camshaft mechanisms and gears were eliminated. On land,...

Incorrect installation dimension

Seal faces loaded with over-compressed spring tension, can generate too much heat. This will damage the o-ring and even fracture the seal laces from thermal expansion (Figure 14-13). If the spring tension is inadequate, the laces can leak after a short while as the softer face wears against the harder face, and the spring tension relaxes completely. The installation dimension is critical to long seal life. Most seal companies provide a set of instructions and engineering schematic with each...

Synthetic fibers

Since the beginning of time, water, either fresh or saltwater, was the only liquid to be pumped, and sealed with stopa or packing in large quantities. Other liquids like milk, beer, paints, oil, gasoline, solvents, tanning acid and medications were made in bottles, jars and barrels and carried from one place to another. With the capacity to move large quantities of fluid with motors and small centrifugal pumps, our forefathers recognized the advantages of expanding production of other liquids...

The turbo tank

Generates its own forced flow with a submersible centrifugal pump. 3. Is ideal for surviving operational problems like Operation away from the BEP on the curve 4. Is good as an environmental control (to reduce heat, dissolve crystals, absorb gases). 5. Is ideal as the installed spare. 6. Continues cooling independent of the process pump. 7. Is not location specific in relation to the seal. 8. Is not a sealed pressure vessel.

Contents

NPSH, Net Positive Suction Head 12 Definition of NPSHr (required) 13 Definition NPSHa (available) 14 The effects of vapor pressure on pump performance 25 Cavitation A practical discussion 26 Review for preventing cavitation 35 Do something about cavitation 37 4 The Affinity Laws 39 Introduction 39 The Laws 39 The Affinity Laws and the impeller diameter 42 What's the practical application of these laws 42 5 Useful Work and Pump Efficiency 44 Factors that affect the efficiency 48 Impeller between...

Interpreting the evidence

Let's interpret the physical evidence that you might see at these close tolerances and their source. To begin 1. You might see gouge or wear marks all around the circumferences of close tolerances on the rotary elements, and a corresponding wear spot at approximately 60 from the cutwater on the stationary elements. This would be induced or caused by operation, if the plant operators strangle valves to control production flow. Any other discharge flow restriction (clogged filter, pipe...

The elements of the Total Dynamic head TDH

The Total Dynamic Head (TDH) of each and every pumping system is composed of up to four heads or pressures. Not all systems contain all four heads. Some contain less than four. They are I. Hs - the static head, or the change in elevation of the liquid across the system. It is the difference in the liquid surface level at the suction source or vessel, subtracted from the liquid surface level where the pump deposits the liquid. The Hs is measured in feet of elevation change. Some systems do not...

Vaporization cavitation

Vaporization cavitation represents about 70 of all cavitation. Sometimes it's called 'classic cavitation'. At what temperature does water boil Well, this depends on the pressure. Water will boil if the temperature is high enough. Water will boil if the pressure is low enough. According to Bernoulli's Law, when velocity goes up, pressure goes down. This was explained in Chapter 1. A centrifugal pump works by acceleration and imparting velocity to the liquid in the eye of the impeller. Under the...

CTS Index

Absolute pressure (psia), 4 Affinity Laws, The, 39, 98, 99, 235 impeller diameter, 42 air aspiration, 33 alignment techniques, 145 laser alignment, 146 reverse dial indicator alignment, 145 rim and face alignment, 146 American National Standards Institute (ANSI) pump, 58, 138 American Petroleum Institute (API) pump, 59 American Voluntary Standards (AVS), 58 Archimedcs, 77 atmospheric pressure (ATM), 4 ballbearing, 155 roller bearing, 156 bearing temperature, 165 Bernoulli's Law Principle, 3,...

The sweet zone

Consider the following graph, Figure 9-14. Radial loading on the shaft rises if the pump is operated too far to the left or right of the best efficiency zone. Another interpretation of the same concept is to say that the maintenance and problems rise when the pump is operated away from its BEP. Many pumps have a rather narrow operational window. These pumps can be very efficient if they are correctly specified and operated. This is discussed completely in Chapters 7 and 8 Pump Curves and System...

Larry Bachus

Larry has almost 30-years experience in maintenance with industrial pumps. His areas of expertise include diagnosing pump problems and seal failures. Larry is highly regarded for his hands-on personalized consulting. He speaks fluent English and Spanish. He has taught pump and seal improvement courses all over the world. His investigations into pump failure have led to inventions, tools and devices used in the chemical process industry. He is an active member of American Society of Mechanical...

The Submergence Laws

Fluid Velocity in Suction Pipe in Feet per Second, g Fluid Velocity in Suction Pipe in Feet per Second, g Use a 'Y' branch and not a 'T' branch. This will reduce turbulence. Don't drain from the bottom of the tank Don't drain from the bottom of the tank Correct suction piping leading to the pump Figure 17-15 . You should respect 10 pipe diameters before the first elbow in the suction piping Figure 17-16 . Example If the pump has a 6 inch suction nozzle, you should have 60 inches of straight...

Alignment techniques

Centrifugal Pump Radial Axial Alignment

There are a variety of shaft alignment procedures. The configuration and size of the equipment determines the best alignment method. Generally the driver or motor should be aligned to the pump. The motor shaft centerline should be shorter and brought up to the pump shaft centerline with shims or spacers. The pump is generally fixed and attached to the suction and discharge piping, so it is almost impossible to move. The volute casing aids in supporting the piping, so it should be fixed to a...

Coupling alignment

Don't use a flexible coupling to compensate for misalignment between the pump and motor shafts. The purpose of the flexible coupling is to compensate for temperature changes and to permit some axial movement of the shafts without interference, while they transfer energy from the motor to the pump. There should be enough space between the coupling halves so that they don't touch should the motor shaft move forward toward the pump. This space should also consider movement due to wear in the pump...

Advantages of ORings

The majority of mechanical seal manufacturers make models that incorporate o-rings as secondary seals. These o-rings offer advantages over other forms of secondary elastomers. 1. The O-ring can flex and roll - There is no need for the shaft to slide and rub under the o-ring while under an axial load. 2. Availability - It's the only elastomeric seal form easily available in most cities and towns. 3. Material of construction - Almost all elastomeric compounds are available in the o-ring...

The lantern ring

The lantern ring performs three basie functions 1. To supply new lubricant to the packing rings, which is normally lost in operation. 2. To supply 'back pressure', which aids in impeding the entrance of abrasive and corrosive material into the stuffing box. Abrasives and corrosives will damage the shaft or sleeve, and disintegrate the packing. 3. To cool the packing and shaft and dissipate the heat generated between these members, which will increase the normal service life of the packing and...

Definition of NPSHr required

It is the energy in the liquid required to overcome the friction losses from the suction nozzle to the eve of the impeller without causing vaporization. It is a characteristic of the pump and is indicated on the pump's curve. It varies by design, size, and the operating conditions. It is determined by a lift test, producing a negative pressure in inches of mercury and converted into feet of required NPSH. An easy way to understand NPSHr is to call it the minimum suction pressure necessary to...