A leak rate of one drip per minute equals about a liter (quart) of water in three days. For an O-ring on a submersible pump, that is a major problem. O-rings are often used for static seals on submersible pumps. Understanding what makes a good static seal and what causes one to leak is important for pump maintenance and repair technicians.
The effectiveness of an O-ring static seal depends on the dimensions of the mating parts, the surface finish of the mating parts and the characteristics of the elastomeric O-ring. The focus of this article is the dimensions and condition of the mating parts.
The dimensions (geometry) of the mating parts are important for achieving a good O-ring seal and will not change unless the surfaces have been corroded, damaged or machined. The radial mounting’s inside diameter (ID), for instance, will be slightly larger than the O-ring’s ID. Therefore, the O-ring will be subjected to a slight stretch when slipped into the mounting groove.
To form a leak-free static seal, the O-ring should be compressed about 40 percent by the mating parts. The amount of compression is a function of the mounting groove’s depth (GD) and the O-ring’s cross-sectional diameter (OCD). The width of the mounting groove (GW) must allow the O-ring to expand in that dimension while it is being compressed in the depth dimension (see Figure 1).
Figure 1. O-ring groove and cross-sectional diameter dimensions
Original equipment manufacturers’ (OEM) kits may contain unnecessary parts that do not match the pump being repaired because they typically provide O-rings for several different models. If an O-ring fits loosely on the mounting ID or must be stretched significantly, it is probably not the correct O-ring.
If the O-ring groove has been re-machined, the dimensions and proper compression should be verified. Table 1 provides a few common O-ring dimensions, but some O-ring materials, such as polytetrafluoroethylene (PTFE), require different compression and dimensions.
Table 1. Common seat dimensions for standard O-ring sizes
Identification of Critical Surfaces
O-rings on submersible pumps may be compressed radially between the ID and outside diameter (OD) of the O-ring (radial mount), or axially between the “top” and “bottom” of the O-ring (face mount). To obtain a leak-free static seal, identifying the critical surfaces that apply the compression is important. The surfaces should have no nicks, scratches, deformations, tooling marks or corrosion. They should have a surface finish of 0.8 micrometers (32 micro inches) root mean square, which will show a dull sheen.
For example, a common O-ring application for submersible pumps is sealing the stator end brackets. Usually, the O-ring fits around the end bracket rabbet and is compressed between the machined faces of the rabbet shoulder and the stator. This forms a seal between these two mating surfaces, not on the OD of the end bracket rabbet. However, some stators have a recess in the rabbet, so the O-ring “disappears” when the end bracket is in place. In that case, the seal is radial—between the stator ID and the end bracket OD.
The elastomers used for O-rings will lose their elasticity with time and when exposed to heat. While installing O-rings to their proper compression for a short time will not cause damage, replacing the O-rings with new ones when repairing a pump is recommended as a best practice.
The condition of the mating parts can compromise a static seal. For example, a submersible pump failed after a year of service because of moisture intrusion (the stator was full of water and failed to ground). When the repaired unit failed again after three months, careful inspection of the O-ring fit at the end bracket rabbets revealed that poor machine work had caused the original failure. Once the pumpage had bypassed the O-ring seal, it corroded the sealing surface, causing the leakage to accelerate after the repair.
When properly and uniformly compressed against flat, smooth sealing surfaces, O-rings make great static seals for submersible pumps. Careful inspection of compression surfaces and attention to proper O-ring fit will help ensure leak-free repairs.