ON HYDROSTATIC TESTING HEAT EXCHANGERS.pdf
ON HYDROSTATIC TESTING HEAT EXCHANGERS INTRODUCTION Section VIII Division 1 of The ASME Boiler and Pressure Vessel Code (Code)i[i] requires testing the integrity of pressure vessels by subjecting them to a hydrostatic pressure test of 1.3 times the maximum allowable working pressure (MAWP) corrected for the difference between allowable stress at the test temperature and the design temperature. The Code provides for testing at higher hydrostatic test pressures but most pressure tests are at the 1.3 multiple. For shell-and-tube heat exchangers built to the TEMA Standards, Paragraph RCB1.31 Standard Test, the holding period is at least 30 minutes. The TEMA Standards require testing the shell and tube sides separately in such a manner that leaks at the tube joints can be detected at least from one side. The Code does not permit any visible leakage through the joints during the Authorized Inspector’s (AI’s) examination during and after the holding period on hydrostatic test pressure. Paragraph UG-99(g) states in part, “Following the application of the hydrostatic test pressure, an inspection shall be made of all joints and connections. This inspection shall be made at a pressure not less than the test pressure divided by 1.3. Except for leakage that might occur at temporary test closures for those openings intended for welded connections, leakage is not allowed at the time of the required visual inspection.” (Italics added.) This means that the AI must reject any visible leakage of exposed tube-totubesheet joints such as weeping around the tube-to-tubesheet connections during testing. Inspection is with the shell side of the exchanger under the hydrostatic test pressure divided by 1.3 and the tube side at atmospheric pressure with the tube-to-tubesheet joints visible. Tube-to-tubesheet joints of some types of shell-and-tube heat exchangers, such as fixed tubesheet designs and closed feedwater heaters in which the channel is welded to the shell, are visible only from the channel side with the shell side under pressure and the channel side at atmospheric pressure.
The ASME Code’s Paragraph U-99(g) states, “The visual inspection of joints and connections for leaks at the test pressure divided by 1.3 may be waived: provided (1) a suitable gas leak test is applied; (2) substitution of the gas leak test is by agreement reached between the Manufacturer and Inspector; (3) all welded seams which will be hidden by assembly be given a visual examinations for workmanship prior to assembly; and (4) the vessel does not contain a “lethal substance”. This is particularly pertinent for heat exchangers in lethal service because literal interpretation of the waiver would preclude using fixed tubesheet exchangers for lethal service applications. But many such exchangers have been Code stamped and this practice continues throughout the industry. Users and designers should be aware that the purpose of the Code’s standard hydrostatic test described in Paragraph UG-99 is to test the capacity of the vessel to withstand the design pressure; it is not to determine whether the tubeto-tubesheet joints are sufficiently leak tight to prevent leakage of process fluids with low viscosities through the joints. (Leak rates vary inversely with fluid viscosity.) Users must recognize the hazards of process fluids leaking from the channel into the shell and where hazards exist, specify further leak testing as described in Section V of the Code. This includes the possibility of wiredrawing (wormholing) in high pressure closed feedwater heaters in which a leak of high pressure feedwater through the tube-to-tubesheet welds can erode the steel under the weld overlay to which the tubes are welded. Severe wormholing can shorten feedwater heater life. Current practice is for Manufacturers to use loss of pressure in the channel to determine whether there is leakage from the channel side to the shell side when the shell side is not visible for inspection. However, the gradations on pressure test equipment in common uses are too coarse to indicate very small leaks or weeping. It follows that users should specify that Manufacturers use other methods to verify non-visible leakage when such leaks could be hazardous or harmful during operation of the heat exchanger. See Table 1 for typical test pressures and pressure gage graduations.
Because the ASME Code rules and the API and TEMA Standards do not require gas leak testing to verify that there are no-leaks from the channel side to the shell side, Manufacturers will not perform such tests unless the User or the User’s Agent specifies that they do so. Therefore, when they hydrostatically test the tube side, they rely on pressure loss in the tubes and channel during the holding period to indicate leakage through the tube-to-tubesheet joints. The gages in widespread use in the heat exchanger and pressure vessel industries for hydrostatic testing to meet the Code requirements are dial type Bourdon tube gages. Some Manufacturers use strip chart or circular recording gages but the graduations are similar to those of dial gages. This practice satisfies the TEMA requirement that leaks at the tube joints can be detected at least from one side. API 660 also accepts this practice. Although gas leak testing is not onerous and costly, heat exchanger manufacture is a very competitive business and it is not likely that Manufacturers will perform testing that the ASME Code does not require unless the procurement specification requires it. For constructions in which the Authorized Inspector cannot visibly examine the shell sides of tubesheets, heat exchanger users are cautioned that pressure loss to determine whether there are leaks from the channel into the shell does not indicate weeping through the tube-to-tubesheet joints because the gages in common use are not sufficiently sensitive to indicate a pressure loss that discloses such small leaks. This is especially so when the tube side hydrostatic test pressure is substantially higher than that of the shell side. An analysis of the pressure testing process which hydrostatic test water is applied in the tubeside of an exchanger where the backside of the tubesheet is not visible was adapted from material previously published on the MGT Inc. website. It demonstrates that relying on gage indication of pressure loss to assess leaks (weeping) that would not be visible during hydrostatic testing does not indicate whether there are such small leaks. It shows that using test gage pressure loss to
determine if there is leakage through the tube joints from the channel to the shell of heat exchangers in which the back side of the tubesheet is not visible does not disclose weeping leakage from the channel into the shell through the tube-to-tubesheet joints. Such leakage does not comport with the implication of the language of the Code’s Paragraph U-2(g) that leakage is not permitted. In this discussion weeping is defined as a leak of 20 drops per hour or approximately 1 cm3 (0.061 in3) which if visible would be 10 drops of water on the tubesheet face after the half-hour TEMA minimum holding period. If the exchanger service is for a fluid less viscous than water the likelihood of leakage in services may be very high if the Manufacturer relies on changes in the pressure gage reading to assess whether there is leakage from the channel side into the shell during hydrostatic testing. ISSUES ABOUT HYDROSTATIC TESTING HEAT EXCHANGERS 1. Should the User or the User’s agent or a Manufacturer with knowledge about the hazards of the service be required to specify further leak testing and the kind of leak testing the Manufacturer must use and whether the API-660
requirements? 2. Despite the Code rules about the unacceptability of leakage through the joints of pressure vessels and heat exchangers, the TEMA and API standards do not require leak testing every heat exchanger and Manufacturers will not perform such leak tests unless Users request them. It is necessary to understand the service of the exchanger and the degree of hazard such leakage presents. When determining whether to require the Manufacturer to perform leak tests. For example, a minor tube joint leak in a water-to-water cooler presents no hazard, whereas leakage of a volatile, flammable or explosive fluid could cause damage such as wire drawing, injury or death. How should users deal with the possibility of such leaks?
3. What constitutes a suitable leak test if the standard Code hydrostatic test cannot disclose very small leaks (weeping)? 4. Are only leak tests described in the Code’s Section V acceptable for meeting the Code’s no-leak requirement? The simplest and least costly additional test is gas-bubble testing (often erroneously described as soap bubble testing.) Typically, the Manufacturer pressurizes the shell with air or nitrogen at 30 to 50 psi and applies a commercial bubble former to the tube-to-tubesheet joints. This is a very effective way to disclose leaking tube-to-tubesheet joints. When the User or User’s agent or knowledgeable Manufacturer is aware of the potential hazard of a leak in service, or when the channel side design pressure is substantially higher than that of the shell side, it is reasonable to specify halogen or helium sniffer leak testing to satisfy a no-leak requirements. For tubes welded to the tubesheet and subsequently expanded, the prudence suggests that in addition to such leak testing the welds should be fluid penetrant examined. When the channel side design pressure is very high – in the order of 1500 lb/in2 and above - it is prudent to require cycle testing. Typical procurement specifications for high-pressure feedwater heaters require 10 cycles of bringing the channel to the hydrostatic test pressure followed by dropping the pressure to atmospheric for each cycle. (Figure 1) Such testing has beneficial effects on the structure in addition to possibly cracking subsurface porosity bubbles and disclosing cracks in the welds. For such equipment small leaks of high-pressure feedwater through the tube-to-tubesheet joints leads to wire drawing (wormholing), which when severe can be extremely expensive to repair, especially when considering the loss of cycle efficiency when the heater is bypassed to allow access for repairs. Therefore, typical feedwater heater procurement specifications require helium leak sniffer testing (mass spectrometer) testing the tube-to-tubesheet joints during manufacture.
COMMENTS AND RECOMMENDATIONS
1. The Code is a pressure containment safety code and the hydrostatic test represents only a test adequate for the typical heat exchanger not in a specific service where leakage is an issue. Users should be aware of these facts. 2. Loss of test gage pressure during ASME Code required hydrostatic testing does not disclose very small leakage (weeping) from the channel side to the shell side because of the insensitivity of the test gages used industry wide for hydrostatic testing. 3. Users of the Code should also be aware that, although the TEMA Standards require a minimum of one-half hour hold time of hydrostatic pressure, the Code does not specify a hold time, which would be important for detecting leakage through welds and joints. Because hold time does not guarantee that a joint is 100% free from leaks, Designers, Users, and Manufacturers need to agree on the suitable test(s) for the service conditions. 4. Designers, Users, and Manufacturers should agree on the definition of joint type and to the nondestructive Tests (NDT) for all welded joints. 5. Designers, Users and Manufacturers of heat exchangers should consider the specific language of the Code’s no-leak requirement of UG-99(g) and the differences between it and the requirements of API 660 and the TEMA Standards. 6. When preparing procurement specifications, Engineers should consider the service of the exchanger and the UG-99(g)’s requirements and determine the appropriate leak tests for meeting them. Users and Manufacturers should agree beforehand on the suitable leak detection system when invoking the waiver provisions of the Code’s Paragraph UG99(2)(g). 7. The language of the waiver in UG-99(2)(g) needs clarification with respect to using fixed tubesheet exchangers for lethal service, possibly with a specific exception allowing their use with appropriate precautions.