THE TUTORIALS

NOTE: Some of the blue links do not work on this version of The Tutorials. They only work on the CD version

In the following section we'll be looking at overviews of several pump and seal subjects:

In these tutorials I am attempting to put each of the subjects into perspective. You'll use the tutorials for multiple purposes:

I suggest that you read the entire tutorials and then go back and look up the details of any unfamiliar words or subjects. Any word or phrase in blue and underlined is a link to a detailed explanation of the subject (The links only work on the CD version) Most of the time I have tried to use the link only for the first mention of the word or phrase; otherwise the narrative would be full of links.

CENTRIFUGAL PUMP SELECTION. HOW TO PICK THE CORRECT SIZE PUMP FOR YOUR APPLICATION.

We'll begin by deciding what operating conditions our pump has to meet and then we'll approach pump suppliers to see how closely they can satisfy these needs. Unfortunately no comprehensive theory which would permit the complete hydrodynamic design of a centrifugal pump has evolved in the many years that pumps have been around, so the pump manufacturer will be doing the best he can with the information you supply to him.

To clearly define the capacity and pressure needs of our system we'll construct a type of graph called a system curve. This system curve will then be given to the pump suppliers and they will try to match it with a pump curve that satisfies these needs as closely as possible.

To start the construction of the system curve I'll assume you want to pump some fluid from point "A" to point "B". To do that efficiently you must make a couple of decisions:

Since we're just getting into the subject, one of the first things we should learn is that centrifugal pump people do not use the word pressure. As mentioned in an earlier paragraph they substitute the word "head", so you'll have to calculate the three kinds of head that will be combined together to give you the total head of the system needed to deliver the required capacity. Here are the three kinds of head you'll be calculating:

You'll be calculating these heads on both the suction and discharge side of the pump. To get the total head you'll subtract the suction head from the discharge head and that will be the head that the pump must produce to satisfy the application. It'll become obvious in the calculations, but I should mention here, that if the suction head is a negative number, the suction and discharge heads will be added together to get the total head. If you subtract a minus number from a positive number you must add the numbers together. As an example: 4 - (-2) = + 6

The total head of a pump seldom remains static. There are a number of factors that can change the head of a pump while it's operating, and you should become familiar with most of them.

All of this head information is calculated from piping, valve, and fitting, along with friction graphs you'll find in the index. This head data will be plotted on a set of coordinates called a system curve. Since we'll not be operating at a single point all of the time, we'll make the calculations for a range of different capacities and heads that we might expect to encounter. This range is described as the operating window we need to satisfy the application.

Making these calculations is not an exact science because the piping is seldom new; pipe inside diameters are not exact, and the graphs you'll be consulting cannot compensate for corrosion and any solids built up on the piping, valve and fitting walls.

Life is never simple. This is the point where most people start adding in safety factors to compensate for some of the unknowns. These safety factors will almost always guarantee the selection of an oversized pump that will run off of its best efficiency point (BEP) most of the time.

The final calculations are then plotted on the system curve that describes what the pump has to do to satisfy the requirements of the application. You can learn to do all of this by referencing the following subjects:

The pump manufacturer requires a certain amount of net positive suction head required (NPSHR) to prevent the pump from cavitating. He shows that number on his pump curve. When you look at the curve you'll also note that the net positive suction head required (NPSHR) increases with any increase in the pump's capacity.

You'll also be calculating the net positive suction head available (NPSHA) to be sure that the pump you select will not cavitate. Cavitation is caused by cavities or bubbles in the fluid collapsing on the impeller and volute. In the pump business we recognize several different types of cavitation. :

Pump cavitation is recognized in several different ways

Remember that the net positive suction head required (NPSHR) number shown on the pump curve is for fresh water at 68° Fahrenheit (20°C) and not the fluid or combinations of fluids you'll be pumping.

When you make your calculations for net positive suction head available (NPSHA) the formula you'll be using will adjust for the specific gravity of your fluid.

When the pump supplier has all of this in-exact information in his possession he can then hopefully select the correct size pump and driver for the job. Since he wants to quote a competitive price, he is now going to make some critical decisions:

He might begin with the type of pump he'll recommend:

There are additional decisions that have to be made about the type of pump the supplier will recommend:

There are multiple decisions to be made about the impeller selection and not all pump suppliers are qualified to make them:

Either you or the supplier must select the correct size electric motor, or some other type of driver for the pump. The decision will be dictated by the specific gravity of the liquid you'll be pumping along with the specific gravity of any cleaners or solvents that might be flushed through the lines. The selection will also be influenced by how far you'll venture off the best efficiency point (BEP) on the capacity side of the pump curve. If this number is under-estimated there is a danger of burning out some electric motors.

If all of the above decisions were made correctly, the pump supplier will place his pump curve on top of your system curve and the required operating window will fall within the pump's operating window on either side of the best efficiency point (BEP). Additionally, the motor will not overheat and the pump should not cavitate.

If the decisions were made incorrectly the pump will operate where the pump and system curves intersect and that will not be close to, or at the best efficiency point, producing radial impeller loading problems that will cause shaft deflection, resulting in premature seal and bearing failures. Needless to say the motor or driver will be adversely affected also.

With few exceptions pump manufacturers are generally not involved in mechanical sealing. You'll probably be contacting separate seal suppliers for their recommendation about the mechanical seal.

Mergers between pump and seal companies unfortunately does not produce the instant expertise we would like sales and service people to posses.

CENTRIFUGAL PUMP INSTALLATION

Some one has to install the pump and all of its associated hardware. The quality of this pump and driver installation will have a major affect on the performance and reliability of the pump, especially if it's equipped with a mechanical seal.

The pump will be installed on a baseplate. The baseplate will be attached to a foundation and grout will be placed between the baseplate and the foundation to transmit any vibrations from the pump to the foundation.

Once the pump and driver are firmly on the foundation it'll be time to connect the piping. Be sure to pipe from the pump to the pipe rack and not the other way, so as to avoid pipe strain that will interfere with the operation of the mechanical seal and bearings.

There are many piping recommendations that you should be familiar with. The leveling, and pump to driver alignment can be made at this point, but you should check the alignment after the pump has come up to its operating temperature because metal parts expand and contract with a change in temperature.

If this is a new piping system some people like to install packing in the pump and run on packing until the new piping has been cleaned of slag or any junk that might be left in the piping system. If it's not a new installation, and there is a mechanical seal in the stuffing box, then installing the mechanical seal environmental controls will come next.

If the pump has an open or semi-open impeller it's time to make the initial impeller clearance setting. The final clearance can be set when the pump comes up to its operating temperature. It's important to note that if you do not have a cartridge seal installed in the pump the seal face loading will change as you make both the initial and subsequent impeller settings and there is nothing you can do about it.

You'll now want to do a proper venting of the pump. If it's a vertical installation you'll have to pay particular attention to keeping air vented from the stuffing box while the pump is running and be sure to vent the space between dual seals if they've been installed.

After you have done all of the above, it's time to check out the mechanical seal environmental controls to be sure they're working properly. In most cases the environmental control will continue to run after the pump has stopped. Be sure the operators understand this or they might be tempted to shut the control off when the pump is between batches. Seal quench is always a problem with operators because the steam or water dripping out of the seal gland looks like the seal is leaking.

A constant monitoring of the pump is a good idea. Are you familiar with some of the more popular monitoring methods? Unlike vibration analysis, monitoring can tell you if some part of the pump is getting into trouble before the vibration starts.

CENTRIFUGAL PUMP MODIFICATION

If you find that your present centrifugal pump is not satisfying the application and running as trouble free as you would like, and you have checked:

Then you may have to purchase a different centrifugal pump, or you might want to consider modifying the existing pump to get the performance and reliability you are looking for.

Here are a few modifications and pump upgrades you can consider:

MECHANICAL SEAL SELECTION

Whenever I use the word fluid, I am talking about either a liquid or a gas. If I say either liquid or gas, I am limiting my discussion to that one phase of the fluid.

Any discussion of mechanical face seals requires that you have many different types of knowledge. The first is, "should you be converting packed pumps to a mechanical seal?" Seals cost a lot more money than conventional packing and unless you're using split seals, they can be a lot more difficult to install. There is a packing conversion down side.

Assuming you have made the decision that the mechanical seal is your best choice for sealing, you must know how to select the correct design for your application. There are many different kinds of seals to choose from:

There are some very desirable design features that you should specify for your mechanical seals:

We'd like to be able to install the seal without having to modify the pump. The seal should be the shortest, thinnest design that'll satisfy all of the operating conditions. Once you have the shortest, thinnest design that'll satisfy the operating conditions, there is seldom a need to modify any seal design.

The specific sealing application will dictate which seal design you should choose. If your seal application falls within the following parameters any stationary or rotating, "off the shelf" balanced, o-ring seal should be able to handle the application without any serious problems:

You may have to go to a special seal design if your application falls into any of the following categories:

If any of the following are part of the application, you may need a metal bellows design that eliminates all elastomers.

You should go to a dual seal application if your product falls into any of the following categories:

You need dual seals as a protection for personnel in the area if your product is any of the following categories:

The other places we use dual seals are:

In the Sealing Application section you'll learn:

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