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).
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.
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.