Gene Vogel
EASA Pump and Vibration Specialist

One drip per minute amounts to about a liter (quart) of water in 3 days. If that’s the leak rate for an O-ring on a submersible pump, that’s a big problem. O-rings are common for static seals on submersible pumps. An understanding of what makes a good static seal, and what causes them to leak, is obviously important for pump repair technicians.

There are three important components of an O-ring static seal: the geometry of the mating parts, the surface finish of the mating parts and the characteristics of the elastomeric O-ring. As long as original equipment manufacturer (OEM) O-rings are used, their characteristics are not a concern. If they are not OEM O-rings, we would need a lot more space to cover that subject. So we’ll focus on the condition of the mating parts.

Importance of dimensions
The dimensions (geometry) of the mating parts are important to achieve a good O-ring seal. The O-ring should be subjected to a slight stretch onto the inside diameter (ID), and should be compressed about 40% by the mating parts. Of course O-ring groove dimensions won’t change unless there has been some significant machine work done to the part. But typical OEM kits often contain O-rings for several different pumps, so there will be extra parts that don’t match the pump being repaired. If an O-ring fits loosely on the mounting ID, or has to be stretched significantly, it probably is not the correct O-ring.

The amount of compression is a factor of the O-ring cross sectional diameter and the depth (GD) of the mounting groove. The width (GW) of the mounting groove must allow for the expansion of the O-ring in that dimension, as it is compressed by the depth dimension. See Figure 1.  

If the O-ring groove has been re-machined, the dimensions and proper compression should be verified. Table 1 provides some common O-ring dimensions, but be aware that some O-ring materials, such as PTFE (Teflon™) require different compression and dimension than those shown.

O-rings on submersible pumps may be radially compressed (radial mount) or axially compressed (face mount).  Radial compression is applied between the ID and outside diameter (OD) of the O-ring; axial compression is applied between “top” and “bottom” of the O-ring. So the critical surfaces are those that apply the compression. These surfaces should have a surface finish of 0.8 µm (32 µin) rms. That’s a dull sheen with no visible evidence of surface scratches or tooling marks. 

Most importantly, the surfaces must be free of nicks, scratches, deformations and corrosion. Be sure to identify which surfaces are applying the compression – those are the critical surfaces. For instance, a common O-ring application on submersible pumps is sealing the stator end brackets. 

Commonly, the O-ring fits around the end bracket rabbet and is compressed between the machined face of the rabbet shoulder and the mating machined face of the stator. The seal is formed on these two surfaces, not on the OD of the end bracket rabbet. But if the stator has a recess cut in the rabbet so that the O-ring “disappears” when the end bracket is in place, then the seal is radial, between the stator OD and the end bracket ID.

The elastomers used for O-rings will lose their elasticity with time and with heat. While simply installing an O-ring to its proper compression for a short time will not damage an O-ring, it is always recommended to replace O-rings with new ones when repairing a pump. 

A case history
A submersible pump was found to have failed after a year of service because of “moisture intrusion.” That means the stator was full of water and had failed to ground. The repaired unit failed in the same manner after 3 months. Careful inspection of the O-ring fit at the end bracket rabbets revealed apparent poor machine work which allowed the original failure. Once pumpage was allowed to bypass the O-ring seal, it caused corrosion to the sealing surface which resulted in accelerated leakage after the repair.

Summary
O-rings make great static seals for submersible pumps – when they are properly and uniformly compressed against flat smooth sealing surfaces. Careful inspection of the compression surfaces and attention to proper O-ring fit will help ensure leak free repairs to submersible pumps.

Categories: Pumps, Pump seals

Tags: Pump seals

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