Active Flow were asked to provide a typical Medium Pressure twin stream Pressure Reducing Station with a minimum inlet pressure of 350mb and 25-50mb outlet (profiled). While the load was quite significant, this wasn’t something that concerned us. The enquiry called for Active/Monitor configuration and we proposed full bore Donkin 680 regulators. This was supplied to the Network in the autumn and was commissioned during the early part of winter.
We were then asked to assist with this site as the regulator outlet pressure was fluctuating as soon as the load changed (increased or decreased). While the technicians could get this to settle and maintain at a stable outlet pressure, this would only do so while the demand remained unchanged.
Active Flow attended site armed with spare pilots (and pilot rails for speed of exchange) and carried out the same checks that the technicians had with the same results. By isolating the profile rail, the regulators became stable. It appeared that the profile rail (inspirator control) was making the regulator too sensitive to the point that hysteresis was experienced that the K pilot/regulator could not recover from. Changes to the settings did not improve matters. A review of the data logger confirmed 1.8 bar was on the inlet during this visit. Questions were asked to its source only to realise that the adjacent HPMP installation feeds the MP network at a steady 1.8 bar all of the time. It was obvious from the low noise levels that the load was fairly low (not a cold day) so the effect of a relatively high inlet pressure combined with a low flow rate meant that these regulators were operating outside of their recommended turn down range, that although in direct acting arrangement were able to control, the boost effect of the inspirator made these regulators oversensitive. In summary, the constant 1.8 bar inlet pressure meant that these regulators would be permanently oversized.
PPS as supplied:
A simple remedy is to fit a snubber in the K pilot’s sense line but as this is not a manufacturer approved fit, it wasn’t accepted or even permitted to try. We know that this solution is used on other Networks across the UK as the oversizing issue is a known problem, but it was still rejected. Another solution was requested.
To help understand the impact of the oversizing (in this case the regulators could supply 4 times the required duty), we went through our sizing program with the project engineer and also offered some suggestions to a possible solution. The quickest solution was to change the 680 regulators for reduced bore versions (like for like exchange and no re-piping). We had these regulators in our stock and got these painted and sent to site within a matter of days. This change did reduce the fluctuation, showing we were on the right lines, but not enough to completely resolve it. We knew this was likely to be the case, and had advised as such, but it was worth a try to prove the point.
Active Flow then proposed the correct size of (smaller) regulators and drew up the new regulator spools that would be required to fit within the now installed PRS to better match the real site conditions. We were also able to support this on the existing welded pipe supports thus saving time on site.
A final proposal was put forward to just change the active regulator and leave the monitor as it is. This gives a cost saving (the monitor is direct acting and did give good control) and means the task on site could be completed quickly.
PRS with new regulator and matching spools:
By having a smaller regulator that is better matched to the site conditions, this restricts the inlet pressure so that the surge of gas that instantly over pressurises the outlet pipework is eliminated and allows the K pilot to work with this regulator to maintain the required (variable) outlet pressure.