Design pressure is usually the pressure set by process engineer, based on the results of a simulation or similar study. Hydrotest pressure is the actual pressure the vessel is tested at. Usually the hydrotest pressure is 1.3 times the design pressure (ASME requirement).
So if the design pressure is 100 KPa(g), the hydrotest pressure would be 130 KPa(g). Therefore the rule (or requirement) is called 10/13 rule.
Maximum allowable Working pressure (MAWP) Vs Design pressure
Design pressure is usually the pressure set by process engineer, based on the results of a simulation or similar study. MAWP is the pressure based on the actual characteristics of the vessel/equipment (which is usually manufactured to exceed the specifications set by process engineer). Maximum allowable working pressure is always greater than or equal to design pressure.
Permissive, interlock - Difference
The PERMISSIVES, are some conditions that need to be satisfied before you can start the machine. For e.g. a compressor can be started only when there is sufficient suction pressure.
INTERLOCK: During the process, if a condition fails, interlocks are activated. For e.g. a pump shutdown interlock is activated when the level of drum goes low.
An interlock can also be a permissive, but the converse is not true. For the compressor example (in Permissives), if insufficient suction pressure is a permissive, the compressor will not shutdown, if there is insufficient suction pressure. It is just a condition for the system to start. But if insufficient suction pressure was an interlock, the system will shutdown, when ever the suction pressure goes below a fixed value. But once shutdown, all the permissives need to be satisfied before the system can start again.
Hot bolting
Hot bolting is a method of replacing the bolts on a live line. Normally done one bolt at a time. Hot bolting should be used only when there is no other reasonable choice.
The criteria typically followed is something like:
• The operating pressure must be less than 75% of the MAWP as allowed under ANSI B16.5 at the operating temp of the piping or process system to be hot bolted.
• The flange must have a minimum of 8 bolts
• The process temperature must b
e between 4°and 71° C.
• All flanges and associated system equipment must be adequately supported, i.e. no excessive vibration or pulsation.
Monitoring for hydrocarbons is also a must during the operation. Generally the procedure for hot bolting is the same sequence as for a tightening operation.
• The gasket area must not show signs of leakage. Piping, flanges, and bolts must not be severely corroded, i.e., to the point of affecting their integrity.
• Existing flange bolts/nuts must be tight and of the correct size and grade.
Hot Bolting" Calculations
I am looking for advice regarding the appropriate calculation method to find the maximum operating pressure we can allow when we perform a Hot Bolt procedure (ie, removing one bolt at a time for maintenace purposes). I can run the calculations using an ASME Section VII calculator (Mr. Pedersen's), but when you reduce the number of bolts by 1, it simply re-distributes the remaining bolts over the diameter. If I remove half the bolts (to accurately reflect the increased distance between adjacent bolts) to get the correct spacing, the bolt stress for MAWP is too conservative as compared to removing only one bolt (I am looking at my limiting factor being bolt stress from either MAWP or seating perspective- whichever is greater).
I have never heard of anyone taking one bolt at a time for maintenance purposes from a pressurised vessel, sounds extremely dangerous to me.
RossABQ - yes, one bolt at a time...but not necessarily "replacing" - we often just remove one bolt, clean it up, put some sort of lubricant on it (the lubricant issue is a whole other can of worms), and then re-install the bolt/nut. We do this on flanges associated with piping AND on vessel manways....most often in preparation for turnaround maintenance, but also on the rare occasions when we find a bolt has some corrosion or when when find a "short bolt" (ie, a bolt that is too short and the nut is not fully engaged...usually on facilites we acquired at one time or another) or when we find a nut or bolt that is not the correct grade....in any of these cases we would then replace the bolt and/or nut. desertfox - I don't believe it is "extrememly dangerous" - the industry has been doing this forever. Most of the larger companies have specific procedures for this operation (I have copies of the BP, ConocoPhillips, and the EEMUA Information sheet for Hot Bolting). Folks, thanx for your interests...and look forward to your thoughts and suggestions for the flange limit calcs.
Hot bolting is fairly common, I've been around it quite a bit before. It's simply replacing the bolts on a live line. You do it a bolt at a time. That being said, hot bolting should be usedo nly when there is no otehr reasonable choice.
The criteria typically followed is something like:
• The operating pressure must be less than 75% of the MAWP as allowed under ANSI B16.5 at the operating temp of the piping or process system to be hot bolted.
• The flange must have a minimum of 8 bolts
• The process temperature must be between 40°and 160° Fahrenheit.
• All flanges and associated system equipment must be adequately supported, i.e. no excessive vibration or pulsation.
Monitoring for hydrocarbosn is also a must during the operation. Generally the procedure for hot bolting is the same sequence as for a tightening operation.
• The gasket area must not show signs of leakage. Piping, flanges, and bolts must not be severely corroded, i.e., to the point of affecting their integrity.
• Existing flange bolts/nuts must be tight and of the correct size and grade.
In my opinion, the answer to your stud bolt question is.... "it depends"....!!!It depends mostly on the cost and degree of corrosion on the bolt. If the bolt is of large diameter and of expensive materials, it pays to be carefull and reuse it. Smaller, more common materials may be replaced as a mater of policy
We have hot bolted at 1250F @ 250 psig. We routinely hot bolt polymer lines at 600F @ 1500 psig. Our process requires that we completely dismantle a production unit at overhaul. This involves the removal of several thousand studs, mainly B-16 5/8"-@ 1/2" but considerable B8 Cl2 material. Part of these production units are components that use H11 SHCS that are changed out and reused on set schedule of approximately 15 days. Each line has 48 of these components that have 98 SHCS. As we have 17 lines the reuse of studs and bolts is almost a necessity. Tough there is not a formal inspection of the studs they are screened by adding 2 nuts to each stud during a process we call Daging, the addition of a collodial graphite lubricant. We run each stud through a Pyrolysis Furnace to remove any existing Dag and the disassembled studs and nuts are run through the Daging bath, Dag and water. After Daging the studs and nuts are assembled with a full nut engagement on each end of the stud. I would say less than 1% are rejected during this process. about 90% of the rejected studs are recovered by lite duty mechanics. It is a very rare occurrence when a mechanic has problem with a stud when the piping is reassembled. Some of the studs in use are over 40 years old as witnessed by some Crane Alloy Studs from the 50's that are found in service. At various times I've removed a sample of studs and physically measured the threads and have never seen anything approaching rejection. There is problem sometime with meaning of "hot bolting" being taken as retorquing a bolted connection at operating conditions. "Hot bolting" and "hot torquing" are two separate operations. Care has to be taken with both operations. I've seen sevral problems with "hot torquing" were people forget the proper tightening sequence.
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