plumbing SAFETY AND RELIEF VALVES PART II

Transcription

1 BY BOB VILLENEUVE SAFETY AND RELIEF VALVES PART II In Safety and Relief Valves Part I in HPAC Magazine s May/June 2005 issue, the terms, definitions and regulations related to safety relief valves were reviewed. The next consideration is properly sizing safety and relief valves. The most important thing to remember when it comes to sizing safety valves is to never select a safety relief valve using the existing pipe size. Always select a safety valve by the required capacity of the boiler, line or system. SELECTION CONSIDERATIONS When a customer calls you for a new safety valve, how do you make sure that he gets the right one? Here are a few questions to ask to get the information required to properly select a safety valve: 1.What is the customer protecting? Is the safety valve going on: a process boiler; a heating boiler; a compressor; a steam line (after a pressure reducing valve, for example); a compressed gas vessel; a pump, and so on? 2.What is the fluid? 3.What is the normal operating pressure of the system and what is the required set pressure for the safety valve? The safety valve should be set at least seven psig, or 10 per cent higher than the operating pressure, to ensure proper seat FIGURE 1 tightness and long life. However, the system should never operate and the safety valve must never be set higher than the vessel s maximum allowable working pressure (MAWP). The MAWP will be found on the vessel s nameplate. 4.What is the required capacity? This is normally found on the boiler s nameplate. If the nameplate is not easily accessible, use the boiler horsepower multiplied by 34.5 (boiler horsepower is equivalent to 34.5 lbs of water evaporated per hour at Standard Atmospheric Pressure and a temperature of 212F) to determine the minimum required capacity in lbs/hr of steam. If it is an electric boiler, multiply the kilowatt rating by 3.5 to obtain the boiler s capacity. If the boiler has a heating surface larger than 500 square feet, two safety valves will be required. For air and gas applications, use the compressor s maximum capacity in SCFM (standard cubic feet per minute). In liquid applications, use the pump s maximum output in US GPM (U.S. gallons per minute). POWER BOILERS As an example, the formula for a 50 hp process boiler with an operating pressure of 100 psig and a maximum allowable working pressure of 150 psig is: 50 hp x 34.5 = 1725 lbs/hr Therefore, the safety valve has a required capacity of 1725 lbs/hr set at a minimum of 110 psig (operating pressure of 100 psig plus 10 per cent) up to a maximum of 150 psig (the boiler s MAWP). Since the safety valve in this case is going to be used on a process boiler, it must conform to ASME Code Section I. In this case, we would choose a valve with an F orifice, equivalent to in 2, which would give us a valve that has a 1 or 1-1/4 inlet with an outlet size of 1-1/4. As a rule, the outlet of a safety valve will either be the same size or larger than the inlet, owing to the expansive qualities of compressed fluids. The chart heading ASME Section I Steam Pounds per hour of Saturated 3 per cent overpressure means that the safety valve will relieve this amount of steam at set pressure plus three per cent. In the example above, the F orifice valve will relieve 1,955 lbs/hr of steam at psig as shown in Figure 1. The same valve used on a steam line (unfired vessel), which is Section VIII of the code, will have a capacity of 2073 lbs/hr because Section VIII of the code calculates the capacity at 10 per cent overpressure, or 121 psig. This illustrates that why and where the valve will be used is just as important to know as what the fluid is and the required capacity. CONTINUED ON PAGE HPAC SEPTEMBER/OCTOBER 2005

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3 CONTINUED FROM PAGE 54 It is important to note that ASME Code Section I allows the reduction of the safety valve port size on a steam boiler as long as the valve can relieve the minimum required capacity. Enlarging the port size for a safety valve on a boiler is not permitted. Valves should have a capacity equal to or higher than the required capacity, but it is not necessary, and sometimes harmful to the equipment, to choose a valve whose capacity This photo was sent to the is much higher than required. author by a contractor in Oversized safety valves often Manitoba, who reported that chatter when they reach set he often sees a disregard for pressure as there is not enough the importance of safety capacity to keep them open valves. and achieve full lift. This chattering can cause serious damage to the safety valve seating surfaces. The vibrations may also damage the equipment that the safety valve is supposed to be protecting. MORE COMPLEX APPLICATION Sizing safety valves for the low pressure side of a steam pressure reducing station is a more involved process. Anytime a steam pressure reducing valve (PRV) is installed there should be a FIGURE 2 Formula to Calculate the Capacity of Pressure Reducing Valves and By-Pass Stations safety valve installed downstream of it to prevent downstream equipment, and the people operating this equipment, from coming in contact with high pressure steam. How is the safety valve to be sized? You need to know the inlet pressure; the outlet pressure (reduced pressure); the pipe size of the pressure reducing valve; if there is a bypass around the PRV, and if so, what is the pipe size of the globe valve; and the flow coefficient of the pressure reducing valve. If all of this information is known, then use Figure 2 with the formula: W = AKC Where: W = Steam flow in lbs/hr through the PRV A = Internal area in in 2 of the PRV inlet pipe K = Flow coefficient for the pressure reducing valve C = Discharge capacity of the orifice, in lbs/hr/in 2 In the real world, the vast majority of the PRV manufacturers do not post the PRV K values in their catalogues. In this situation, use the following formulas: W = 1/3AC - To be used for PRVs without a by-pass line. W = 1/2A1C - To be used for PRVs with a by-pass line. Where: W = Steam flow in lbs/hr through the PRV A = Internal area in in 2 of the PRV inlet pipe A1= Internal area in in 2 of the inlet by-pass pipe C = Discharge capacity of the orifice, in lbs/hr/in 2 Suppose that we have a two-inch PRV that reduces steam pressure from 125 psig to 25 psig. Assume that this system has no by-pass system. To determine the required capacity for the safety valve, we would apply the criteria: W=1/3AC Where (from Figure 2) A = 3.36 and C = 7200 then: W =.33 X 3.36 X 7200 W = 7983 lbs/hr In this case, the safety valve has to have a capacity equal to, or greater than 7,983 lbs/hr set at 32 psig. Remember that the safety valve must be set at least seven psig or 10 per cent, whichever is higher, than the system s operating pressure. Make CONTINUED ON PAGE 58 REFERENCE: A.S.M.E. Code Section VIII, Division 1, UG-133,National Board of Boiler and Pressure Vessel Inspectors,NBIC Code 2004, Appendix G, Table G HPAC SEPTEMBER/OCTOBER 2005

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5 CONTINUED FROM PAGE 56 sure that the safety valve s set pressure is not above the maximum allowable working pressure of the equipment being protected downstream of the PRV. Next, assume that we have the exact same system, but that there is a by-pass system around the PRV with a 1-1/2 globe valve. We must use the other formula (W=1/2A1C) and compare the result with our first calculation. You then must use the higher of the two computed capacities: Where (from Figure 2) A1 = 2.04 and C = 7200 then: W =.50 X 2.04 X 7200 W = 7344 lbs/hr The first calculation produced a required capacity of 7983 lbs/hr. The second calculation produced a required capacity of 7344 lbs/hr, so we must use the higher of the two capacities to choose our safety valve. Had the by-pass valve been even one size bigger, to two inches, it would have changed the required capacity greatly: Where (from Figure 2) A1 = 3.36 and C = 7200 then: W =.50 X 3.36 X 7200 W = 12,096 lbs/hr As shown in Figure 3, a valve with a P orifice (6.692 in 2 ) will be sufficient. WORTH NOTING... The ASME code does not address the selection of safety valves for air/gas or liquid pressure reducing stations. By convention, it is recommended that the safety valve should be able to relieve the maximum amount of flow that the pressure reducing valve is able to put out or relieve the maximum capacity that the compressor or pump that is feeding the system is able to discharge. All safety valves are required to have a test lever when on air/gas or steam, and on water applications over 140F. Heat exchangers fall under the Section VIII Liquid part of the code. The safety valve for a heat exchanger should be chosen by comparing water flow (US GPM) and heat flow (Btuh) and choosing the larger of the two. The valve s capacity should be shown on the nameplate in US GPM. Heat exchangers that operate at a temperature higher than 212F (100C) must have a safety valve selected by the manufacturer of the exchanger. OTHER SERVICE As a rule when sizing safety valves for air or gas service (inert gas only), size the safety valve with a capacity equal or greater than the maximum capacity of the compressor supplying the air or gas to the system. Air/Gas capacities are rated in standard cubic feet per minute or SCFM. If the capacity of the compressor is not known, obtain an estimated capacity by multiplying the compressor s horsepower rating by five (air compressors normally deliver four to five SCFM per horsepower at 100 psig discharge pressure). When sizing relief valves for liquid service, in reference to inert liquids having a viscosity similar to water, the relief valve should be chosen with a rated capacity equal to or greater than the maximum capacity of the pump or circulator. FIGURE 3 LAST LINE OF DEFENCE Sizing safety valves is not rocket science, but it does require a general knowledge of the ASME code and the National Board of Boiler and Pressure Vessel Inspectors (NBIC) inspection code. If you are unsure of how to proceed, any ASME authorized safety valve assembler in Canada can help. Most safety valve manufacturers also have computer-based sizing software available either as a download or by request. Safety valves are known as the last line of defence to help prevent accidents that can kill or maim people and cause heavy property and equipment damage. They should be as important to a maintenance engineer as safety shoes. The fact of the matter is that they are often forgotten or ignored. Local authorities across the country have become more vigilant in their inspections and requirements as to how and when safety valves have to be tested, recertified, and sometimes replaced, to ensure public safety. Rather than think of this as an imposition, consider it a duty to make sure that workplaces, schools, public and private buildings are safe for everyone. Bob Villeneuve is sales manager with Steam & Industrial Equipment (SIE). He can be reached at bob_villeneuve@sie.ca. RATE THE ARTICLE! Will this information be useful? Please circle the appropriate number on the Reader Postcard. Thank you. VERY USEFUL USEFUL NOT USEFUL HPAC SEPTEMBER/OCTOBER 2005