Abstract
Current trends in greenfield natural gas compression facilities and high-pressure gas transmission pipelines are designs for high pressure operation, e.g. PN150. At such high pressures, sound pressure level (SPL) of the noise generated during a planned blowdown can be as high as 165–175 dBA at 1 m distance away from the blowdown stack tip. Even double hearing protection would not be sufficient for an operator opening the blowdown valve at the blowdown assembly even for a short period of time. Blowdown silencers mounted on top of the blowdown stack are limited to the degree of noise suppression they offer (typically of the order of 30 to 40 dB), let alone the requirement for extensive support structures to hold the massive weight on the stack top. One innovation to alleviate this issue, is to place a silencer on a skid on a ground-level pad/support system, at a sufficient distance away from the blowdown stack. In this way, the blowdown valve operator would be exposed to a much lower SPL, as well as the silencer can be as large as desired being skid mounded on the ground. The main issue to be addressed is the dynamic thrust loads That would otherwise be afflicted on the various pipe segments of the piping connecting the blowdown stack top to the location of the blowdown silencer located distance away. Clearly this piping will include several above ground pipe segments, bends, tees and fittings that need to be adequately supported to withstand the dynamic loads as the blowdown valve opens. This paper presents a model approach based on solving the temporal-spatial governing equations in the form of 1-D hyperbolic differential equations, with boundary conditions accounting for the blowdown valve size and type, trim characteristics and opening time, upstream gas pressure and temperature, and downstream restriction equivalent orifice size at the inlet to the blowdown silencer. Results are presented for a case study of a single and dual blowdown assemblies equipped with reduced-bore ball valves (8″ × 6″), with lead piping of NPS 8 joining into NPS 12 header and finally splitting into two ground mounted silencers some 30 meters away. Thrust loads at each segment of these connecting piping were found to depend on whether one or dual blowdown valves are open simultaneously, upstream pressure, and more importantly the location of the choke point (Mach = 1) whether at the blowdown valve itself or at the silencer inlet diffuser orifice. The later was found to be the most critical parameter. The most important objective when designing such a system is to ensure that the choke point is at the silencer inlet orifice to minimize thrust loading on the blowdown valve(s) as well as along the entire lead piping.
