Dirty service

Knockout vessels are the most common form of basic separator. The vessel contains no internals and demisting efficiency is poor. However, they perform well in dirty service conditions (i.e. where sand, water and corrosive products are carried in the well stream). 

Balanced bellows type valves are normally used where the relief valves are piped to a closed flare system and the back-pressure exceeds 10% of the set pressure, where conventional valves can t be used because back-pressure is too high. They are also used in flow lines, multiphase lines, or for ptu affinic or asphaltic crude, where pilot-operated valves can t be used due to possible plugging of the pilot line. An advantage of this type of relief valve is, for corrosive or dirty service, the bellows protects the spring from process fluid. A disadvantage is that the bellows can fatigue, which will allow process fluid to escape through the bonnet. For HjS service, the bonnet vent must be piped to a safe area. 

Another example is a safety valve in standby service. If demands occur very infrequently, time-related stresses such as external corrosion may have a significant influence. Repeated demands in very dirty service could easily lead to faster degradation and failure, whereas repeated demands in lubricated service might actually enhance performance if the failure mode of interest is failure to open. Failure data based on time or demands can also be skewed if the relief valve is initially damaged or installed incorrectly. 

Do not use a double seal in dirty service—the inside seal will hang up. 

Compatible with fouling and dirty service Limitation in pressure/sizes Highly affected by backpressure (set pressure, capacity, stability) Not easily testable in the field... 

The balanced bellows SRV is very effective in corrosive or dirty services because it seals the corrosive or dirty process fluid from contact with the guiding surfaces of the valve, thus preventing sticking as a result of corrosion or ingress of dirt at this contact point. 

Compatible with fouling and dirty service Not easily testable in the field... 

Due to its somewhat more complicated design, in the early days API did not recommend its use on dirty service or polymerizing fluids. However, since then some renowned manufacturers have found solutions to overcome these problems. 

Dirty service with particles in the fluid A spring-loaded valve, preferably with elastomer soft seat is preferred (preferably no PTFE). An elastomer soft seat greatly limits seat damage. PTFE is hard and easily scratched and not good for particle-laden fluids. If a pilot-operated valve is preferred or necessary for the application, the pilot should be protected from the particles. There are different options and configurations to protect pilot valves nowadays, but the option depends on the dirtiness of the fluid. This should be discussed in detail with the SRV vendor (also see Section 5.3.3.4). 

Dirty service (plate columns are easier to clean). 

If capacity and vapor flow variability (i.e., turndown) is not an issue, I use sieve trays with 3/8-inch holes (clean service) or Vi-inch holes (dirty services). Sieve trays have about 10 percent less capacity than grid trays. But anyone can make them in a day (i.e., new tray panels) and the price is minimal. 

Ad Required heat transfer area for dirty service, ft2. 

Uc overall heat transfer coefficient for clean service, btu/hr-ft2- F. Uf overall heat transfer coefficient for dirty service, btu/hr-ft2- F. W fluid flow rate, Ib/hr. 

---