Everything you wanted to know about Wear Rings (but were afraid to ask) Part 1

As humans (and struggling souls), we are afflicted by many cognitive biases. It doesn't much matter what sphere of action we examine - be it family, society, politics, personal development or our professional lives. Before we've got our first morning beverage (for me Lavazza Prontissimo coffee in case you were wondering), we've likely perpetrated at least one of them. (That number rises to >13 for avid practitioners of politics).

With that in mind I'll freely admit that a cognitive bias I wrestle with a lot is The Curse of Knowledge. This bias tends to appear frequently amongst Engineers and Scientists. It generally exhibits itself as the assumption that just because something is clearly understood and "simple" to you, that it is similarly understood and simple to everyone.

With my personal failings unwrapped, that brings us to the subject du jour.

Centrifugal Pump Wear Rings

Those of us who deal with the specification, design, manufacture and troubleshooting of these machines on a daily basis often forget that this simple part belies a lot of complexity as wear rings have major impacts on the functioning of the pump.

I'm going to try my best to due the subject justice, however I'll hedge my bets and plan for it to be a 2 part series. This is because I know from experience I'll receive a number of helpful comments reminding me of gaps in my knowledge.

With that said, and thanking Kirit Domadiya and Dave Goddard for their help, advice and support in creating this article, let's make a start...

Ok, so what is a Centrifugal Pump Wear Ring ?

A centrifugal pump wear ring is a close clearance device present in most (but not all) centrifugal pumps. The vast majority of them are radial clearance devices (we'll talk about other variants in the following paragraph). You will see these fitted on one or both sides on the pump impeller - basically anywhere a pressure differential exists.

The wear ring may be separate replaceable component(s) or it may be integral to the pump impeller and casing. The graphic below shows a OH2 style pump fitted with wear rings on both sides of the impeller in a manner typical for its type.

As I mentioned some types of centrifugal pump do not have wear rings. These include pumps with open impellers, recessed impellers, Barske impellers and regenerative turbine impellers. Note that some of these will have wear surfaces with a close axial or conical clearance providing similar primary functionality as a wear ring. If in doubt ask the pump supplier to confirm what type of close clearance is fitted.

In summary the key characteristics to look for are:

1. A close clearance
2. A surface rotates with the impeller while the counter surface is stationary

The following graphic shows some of the types you may encounter, keeping in mind the vast majority will be the plain cylindrical type shown at the top left.

So why is it called a Wear Ring ???

The term "Wear Ring" or "Wear Surface" is derived from the expectation that over time due to the high velocity fluid passing across it, wear will result. Other factors such as erosion from particles in the fluid, contact between the surfaces and corrosion can also be contributors to wear.

It is normally considered to be a part that will be replaced or refurbished during planned routine maintenance.

What is the purpose of the Wear Ring ?

When we consider radial wear rings they have primary purpose/functionality (which they share with axial and conical wear surfaces) as well as several secondary purposes depending on the pump type.

The primary purpose/functionality is to limit fluid leakage from the high pressure (outlet side) of the impeller to the low pressure side of the impeller - typically the impeller eye. Without the presence of the wear ring, fluid would be free to flow between these regions and the efficiency of the pump would be significantly reduced.

The graphic below illustrates the purpose statement on a OH style pump impeller. I've marked the High and Low pressure regions and the direction of the leakage flow.

That's it - we could stop here and say job done, but there are however a number of secondary functions for the wear ring as well. We will discuss these later on in Part 2 of this series.

What clearances do Wear Rings have ?

Given the previously stated primary purpose, it should be apparent that the smaller the clearance, the less fluid leakage will occur. This creates conflicting requirements for the pump designer.

They want to minimize the clearance in order to achieve maximum pump efficiency. However if the clearance is too small then the movement of the pump rotor under normal operation will result in rubbing, wear and even seizure of the pump. As a result several standards for wear ring clearances have been created over time. Some are embedded in the API 610 standard while others are empirical "rules of thumb" developed by the pump OEMs. To aid comparison I've put them together in the tables below. There are 4 basic minimum wear ring clearances you will encounter:

1. API 610 Table 6 This is the standard API 610 mandated clearances. It is important to note that API 610 was historically written around maximizing reliability rather than maximizing efficiency.
2. API 610 Table 6 + "Hot Clearances" This is the standard tabled clearances + a mandated additional 0.125mm (0.005") when the temperature of the fluid exceeds 260°C (500°F) or when the wear ring materials are "galling"
3. Manufacturers Standard. This is the clearance used by the OEM for pumps where no external specification or standard like API 610 is applied. While every OEM is different this will typically be ≈70% to ≈75% of API 610 Table 6 clearance.
4. Non Metallic. This is the clearance used by the OEM for pumps when efficiency optimization requires the use of non metallic wear rings. Non metallics you will commonly encounter are PEEK, Vespel? and Graphalloy?. We'll talk about why these materials are chosen and the resulting trade offs in Part 2. As with the Manufacturers Standard, every OEM has a different rule, but the clearance will be ≈50% of API 610 Table 6 clearance.

The tables below detail the minimum clearance for of the 4 basic types. Note that some manufacturers may additionally set a floor for clearances on small diameters due to manufacturing tolerances.

What is the effect of the different wear ring clearances ?

Here I'll deal with the effect of different clearances on the primary purpose/functionality - pump efficiency. (The clearance will also affect the secondary characteristics which we'll cover in Part 2).

The crucial thing to understand here is that small low specific speed pumps are most affected by wear ring clearances and large high specific speed pumps are least affected. To understand why this is, let's recap some key concepts:

Pump specific speed (nq). This is defined by the formula where Q is the flow through the impeller, H is the head developed by the impeller and N is the speed of rotation. Pumps developing a high head but a small flow have a low specific speed. Pumps developing a low head but a high flow have a high specific speed.

You can generally identify the specific speed of a centrifugal pump impeller by the shape of the impeller. The diagram below from the Hydraulic Institute gives the typical geometry you can expect to see.

Now considering the primary purpose of the wear ring is to separate high and low pressure regions. It follows that for pumps that develop a lot of head (pressure) but a small flow will be very sensitive to wear ring clearance. The high pressure will push a large amount of flow across the wear ring relative to the amount of flow the pump generates.

Conversely, pumps that develop a low head (pressure) but lots of flow will be relatively insensitive to wear ring clearance.

The physical size of the pump can also contribute to some extent because manufacturing constraints and achievable part tolerances can result in higher relative leakage on small pumps.

The graph below shows how efficiency is affected by the choice of clearances and the pump specific speed relative to "Manufacturer's Standard" clearances.

• Going from "Manufacturer's Standard" clearances to API 610 or API 610 Hot results in a drop in efficiency
• Going from "Manufacturer's Standard" clearances to Non Metallic results in an increase in efficiency

It should be noted that the above graph is the generally expected result for pumps attaining typical efficiencies and pumping clean water or similar viscosity fluids. Pumps with significant design tradeoffs - such as solids handling pumps and pumps handling viscous fluids will differ materially from this.

You should not assume the efficiency change for your pump will be precisely the figure shown here. Ask your pump supplier to provide a performance curve for your precise circumstances.

Computing the cost of your clearance choice

Another good way to look at the effect of changing clearances and pump efficiency is to consider what this delivers in terms of energy cost savings . I discussed this point in the context of pipeline pumps in a past post which you can find here.

If we take a typical pump with 29 metric specific speed (1500 in US units), and create a graph based on 8000 hours per year operation, you can see the yearly incremental energy cost when increasing the clearances and the energy saving when reducing them.

***Note that what is shown is just incremental costs resulting from the wear ring clearance change. A pump in the 2500 kW range is going to burn through $ millions per year in total energy costs***

Even for smallish pumps picking a clearance than is larger than need for operational reliability can significantly impact your life cycle costs given that they will run for 15 to 25 years.

Ok that's all for Part 1. In Part 2 we will discuss the secondary Wear Ring functions, material selection, when to repair a Wear Ring and anything else that comes up in the comments.

As always your comments (whether agreeing or disagreeing), are most gratefully received.

Until next time Beatus Centrifuga