Once the contents and rough shape of a subsea housing are known, how do we select a seal to keep the water out? Here are some tips to start, followed by a bit more discussion:
- Use O-ring whenever possible, other sealing options will be discussed later.O-rings are very forgiving and will work again and again, often years without maintenance. Because of the nature of the design, the O-ring is pushed against the side of the groove and deforms under pressure, causing it to seal even uneven surfaces (so long as they are fairly smooth).
- Buna-N compound generally the best, with a durometer (hardness) of 70A or 90A.There are many, many different materials from which O-rings are made, but the common Buna-N, a resilient rubber, is by far the most common for subsea use. Most other flavors are to deal with corrosive chemicals, extreme heat, or extreme cold. But Buna-N is good for Fresh water, salt water, dirty water, and most hydraulic oils and is also one of the cheapest and most available types of O-ring material on the market. Higher “Durometers” indicate a harder material; 70A is the most common and is a moderately soft material that is good in most applications. 55A is very soft and is only used for low pressures when the ability of the O-ring to move and conform is critical. On the harder end, it is rare to go above a durometer of 90A, which is significantly harder than 70A. 90A is commonly used at high pressures above 5,000 PSI (350BAR) or on annular seals such as a round flange inside a pipe where a more flexible O-ring could be “rolled out” of the grove by friction when installing. The stiffer O-ring is used at higher pressures because it is harder for the fluid pressure to “extrude” it through a small gap between the sealing surfaces. At 10,000 PSI, a soft O-ring, 3/32in in diameter, can be pushed into and even completely through a gap of only a few thousandths of an inch.
- Always refer to the Parker O-ring Handbook for groove dimensions and tips.This excellent and free resource is a go-to for any engineer working with these types of seals and represents a breadth of knowledge few engineers would ever encounter. Unless there is a very specific and well understood reason to deviate from the range of sizes and tolerances provided in the text, stick to what is recommended there.
- Measure the housing and select the closest available size O-ring, then design the grove around the recommended dimensions. Again, the Handbook will come in handy, as there are ANSI industry approved standard sizes. Note that almost all stock O-rings are round, but they can easily be placed into any shape, so long as the radii of corners are not too tight (a few times the diameter of the O-ring, depending on hardness). However, O-rings are sometimes custom molded into specific shapes to match the groove in the housing. While rarely necessary, this is often for ease of manufacture, as trying to hold a round O-ring in a grove of a complex shape often results in the O-ring jumping out just as the housing is sealed, resulting in a cut O-ring, a failed pressure test, and frustrated assembly technician.
- Face sealing O-rings are preferred to corner or annular sealing designs. This is because the ratio of the pressure pushing the parts together, relative to the pressure pushing the seal in to place is so great that minor warps and manufacturing errors in the part melt away as the face of a lid is pushed on to the mating part, even at very low pressures.
- O-rings can be stretched slightly to fit, but they can only be compressed a small amount before they jump out of the groove prior to assembly.When faced with the need to use a stock size O-ring, especially at in large sizes over 8in/203.2mm in diameter, there may be no choice but to get a custom length O-ring. However, one tip when there is enough space available and the seal is only a little too large, is to add some extra twists and turns into the groove. This increases the required diameter and may be enough to make a stock O-ring fit perfectly. If not, McMaster Carr will fabricate a custom length O-ring, in most materials, in 1-2 weeks and for a reasonable price.
- Use ROUND O-ring cross-sections in most cases. The cross-sectional shape of an O-ring can be many shapes, including round, square, X, and there are "backup rings" as well which support the O-ring. Fortunately, a standard "round" style of O-ring works for in most cases. Backup rings are commonly used with Round O-rings to provide support an pressures above 5,000PSI (350Bar). When the application is under 5,000PSI, and there is no significant dynamic movement (such as occurs in a hydraulic piston or rotary fluid slipring) then a round O-ring cross section is all that's needed.
For another 292 pages of everything O-rings, please see the Parker O-ring Handbook below:
https://www.parker.com/content/dam/Parker-com/Literature/O-Ring-Division-Literature/ORD-5700.pdf