1/14/13 Open vs. closed impellers

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Subject: Open vs .closed impeller design pumps 14-2.

The open impeller design

The fluid enters the eye of the impeller where the turning vanes add energy

to the fluid and direct it to the discharge nozzle. A close clearancebetween the vanes and the pump volute, or back plate in a few designs,

prevents most of the fluid from recirculating back to the eye of the

impeller.

(L) shows the leading edge or higher-pressure side of the impeller. (T)

describes the trailing edge of the impeller

The closed impeller design

The fluid enters the eye of the impeller where the vanes add energy to the

fluid and direct it to the discharge nozzle. There is no impeller to volute or

back plate clearance to set.

Wear rings restrict the amount of discharge fluid that recirculates back to

the suction side of the impeller. When this wear ring clearance becomesexcessive the wear rings must be replaced.

Did you get the difference? High pressure always flows to low pressure, so we now have two separate methods

of restricting internal recirculation that can lower the efficiency of your pump and generate a lot of unwanted heat

to the product you're moving.

For the open inmpeller design you have to set the inpeller to casing clearances

For the losed impeller you will need wear rings.

There are advantages and disadvantages to each design:

CLOSED IMPELLER OPEN IMPELLER

Can compensate for shaft thermal growth, but if there is toomuch axial growth the vanes may not line up exactly with the

discharge nozzle.

The impeller to volute or back plate clearance

must be adjusted when the pump is at operating

temperature and all axial thermal growth has

occurred

Good for volatile and explosive fluids because the close

clearance wear rings are the parts that will contact if the shaft

displaces from its centerline

You would have to use soft, non-sparking

materials for the impeller and that is not very

practical.

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The impeller is initially very efficient, but looses its efficiency

as the wear ring clearance increases

Efficiency can be maintained through impeller

clearance adjustment.

No impeller adjustment is possible. Once the wear ringclearances doubles they have to be replaced. This means the

pump had to be disassembled just to check the status of the

wear rings.

The impeller can be adjusted to compensate for

wear and stay close to its best efficiency. No

pump disassembly is necessary.

The impeller can clog if you pump solids or "stringy material".

It's difficult to clean out these solids from between the

shrouds and vanes.

The open impeller is less likely to clog with

solids, but if it does, it is easy to clean.

The impeller is difficult to cast because the internal parts are

hidden and hard to inspect for flaws

The open impeller has all the parts visible, so it's

easy to inspect for wear or damage

The closed impeller is a more complicated and expensivedesign not only because of the impeller, but the additional

wear rings are needed.

The pump is less costly to build with a simple

open impeller design.

The impeller is difficult to modify to improve its performance.

The vanes can easily be cut or filed to increasethe capacity.

The specific speed choices (the shape of the impeller) are

limited

You have a greater range of specific speedchoices.

My experience in Europe has been that about 85% of the pumps used in the process industry are of the closeimpeller design. Here in the United States it is the opposite, with the exception of oil refineries.

At one of my International seminars I quizzed a couple of KSB Pump Company application engineers about this

difference and was told they used closed impellers more often because the German mechanic will not make theproper impeller clearance adjustment.

Oil refineries choose the closed impeller design because their products are often explosive or a fire hazard. If you

use open impeller pumps in these applications there is always the danger of the impeller contacting the volute andcausing sparks. This means that the impellers would have to be manufactured from a non-sparking material,

which is often too soft for the abrasives in an oil refinery application.

If you want to get a feel for the thermal growth involved in a typical pump, be aware that a stainless steel shaftgrows both radially and axially at the rate of 0.001 inch, per inch of shaft, for each 100°F (0.001 mm/millimeter

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of shaft length or diameter/50°C) rise in temperature.

Let's take a look at a typical heat transfer oil pump running at 600°F (300°C) and see what type of expansions

we are talking about. We will start with the inch version and assume a 20-inch long, 1.875-inch diameter shaft. Ifthe 1.875 diameter shaft measured twenty inches from the end of the impeller to the thrust bearing and you

heated the shaft to an average of 400°F over ambient, it would grow 0.080 inches in length and 0.0075 inches indiameter.

This would be enough axial growth to allow the impeller to contact the volute because a typical impeller to

volute clearance would be between 0.015 and 0.020 inches. The volute is often manufactured from adifferent material than the shaft and we have no evidence that both the shaft and volute will grow in the

same direction and at the same rate.The radial growth is enough to allow the shaft to contact the low expansion metal vibration dampers

frequently used in metal bellows seals specified for this service.

If a heat transfer oil pump in the metric system had a 48mm shaft, 450mm long heated to 200°C over ambient, itwould grow 1.8mm in length and 0.20mm in diameter

This is the reason both seal and pump manufacturers recommend turning the shaft by hand prior to start up, butbe careful, it's hot!

All of this means that all impeller clearance must be set when the pump is at operating temperature. It also meansthat you're going to have to specify cartridge mechanical seals in these applications because their operating lengthmust be set when the pump is at operating temperature, or anytime after the open impeller has been adjusted to

compensate for vane or volute wear.

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