Why Do We Do It Like We Always Do It ?
by Karl D. Conley. CIPE
If we were to ask many of our estimators, designers, and inspectors what materials were required to rough in the water to a lavatory, for example, the answers would be rather predictable. Supplies and supply stops, copper tubing, a few ells, straps or brackets, and a pair of tees and air chambers.
Air chambers. Preformed at a factory or field fabricated are intended to prevent water hammer in piping systems. This sounds right, but letís look a little closer at the facts and the rules.
The Illinois Plumbing Code states :
Air chambers. Where an air chamber is installed in a fixture supply, it shall be at least 12 inches in length and at least the same size as the fixture supply. Where an air chamber is installed in a riser, it shall be at least 24 inches in length and at least the same size as the riser.
Mechanical Devices. Where a mechanical device or water hammer arrestor is used, the manufacturers specifications for location and installation shall be followed.
Note that the code requires air chambers or hammer arrestors only when the branch or riser serves fixtures or devices with "quick-opening" valves. The definitions section of the code defines these as " a valve or faucet that closes automatically when released or one that has a fast action closing." This rather vague description is very subject to interpretation, and Iím sure has caused no end of headaches to contractors as well as inspectors.
What is the rest of the country doing about water hammer? ( or hydraulic shock as it is more properly described ) Some codes have eliminated air chambers in favor of mechanical devices. The Uniform code does not require them, but they are permissible. The Uniform Code is the model used in over 1/3 of the states. The "National Standard Plumbing Code" upon which much of the Illinois code is based, permits air chambers ONLY if the air chamber is in a readily accessible location, has an accessible method of draining and a method of reintroducing air into the chamber after it becomes waterlogged. This brings us to the reason that air chambers are falling out of favor in many localities. Most are undersized, most will not absorb the levels of pressure that can be developed in a hydraulic shock conditions, and ALL of them will waterlog, most sooner all later.
To give you some idea of the magnitude of forces involved in hydraulic shock, below is the equation to determine the force:
( taken from Alfred Steeleís "Engineered Plumbing Design" 2nd Ed. Chap.12)
P= the shock pressure in addition to line pressure
w= the specific weight of the fluid in lb./cuft
a= 4660 / (1+KB) Ĺ ( where K is the modulus of elasticity and B isthe ratio of the pipe diameter to itís wall thickness)
V = change in velocity in feet per second
g= the acceleration of gravity
Without solving these equations, I can tell you that the magnitude in most cases will be about 60 times the flow rate. This means that if a Ĺ" line is flowing at 10 feet per second, and a valve is slammed off, the resultant hydraulic shock pressure wave will be around 600 psi plus the beginning line pressure, and it travels back up line at about 4000 to 4500 feet per second.
This applies only if the line is shut down very quickly. The mathematical definition of a Quick-closing valve ( according to Steeleís Engineered Plumbing Design and the American Society of Plumbing Engineersí "Data Book" chapter 3 ) is:
the closing time in seconds is less than
where L = the length of the pipe from itís supply (tee off of main) to the valve causing the shock effect
and a = the same as above which is the pressure wave speed in water @ 4000 to 4500 fps
so a quick closing
valve on a line 50 feet long would need to close faster than
This sounds awfully fast but it really isnít compared to solenoid and pilot operated valves. Dashpot self closing faucets, however, canít shut off anywhere near that speed. Some may feel that they have witnessed water hammer in lines comparable to this , but should remember that water noise and hydraulic shock almost always are caused by the same root design problem, undersized branch lines causing excessive flow rates. Most if not all pipe noise and hydraulic shock will be eliminated by sizing all fixture, branch, and riser piping to flow at maximum demand at speeds less than 10 feet per second, with 5 fps being a design desirable for safe, quiet, long lived installations.
If, however, some method of shock absorption must be employed, the tables developed by F.M. Dawson and A.A. Kalinske of the Iowa Institute of Hydraulic Research in their technical bulletin #3 should be reviewed. These sizes are of sufficient capacity to protect against moderately high pressure waves, however , these tables are for new or freshly renewed air chambers and the bulletin reminds all that the unisolated air chamber will lose itís air and fail relatively soon after installation.
|diameter||length||flow press||flow speed||int volume||phys size|
|1/2"||25'||30psi||10fps||8 cu in||3/4"x15"|
|1/2"||100'||60psi||10fps||60 cu in||1"x69.5"|
|3/4"||50'||60psi||5fps||13 cu in||1"x15"|
|3/4"||200'||30psi||10fps||108 cu in||1.25"x72.5"|
|1"||100'||60psi||5fps||19 cu in||1.25"x12.7|
It can be seen that the currently required sizes are seriously undersized and should be upgraded if they must be used.
Also, considering that few home or building owners would renew the air in chambers even if they knew how, waterlogged chambers would often be in violation of Illinois Code 890.1200 c) "dead ends".
Mechanical shock arrestors, on the other hand, are manufactured to a nationally recognized standard from the Plumbing and Drainage Institute known as PDI - WH201 which also includes a method of sizing these devices. There are 6 sizes and multiple units are used to satisfy heavy demands. Most if not all shock arrestor makers comply to this specification and have adopted this method of sizing as their recommended. They never waterlog, unless they fail catastrophically, but modern methods and materials have made them very reliable.
The time has come to rethink the code position on water hammer and air chambers. Modern design, methods, materials, and practice have made better options available to the designer and contractor. Just as iron replaced lead, and copper replaced iron, so must we be prepared to change for the better at every opportunity. To continue with outmoded methods because "thatís the way weíve always done it" or " Ďcause if itís not in the code, we canít" is a failing we can, as an industry, ill afford.