Process control applications with high turndown requirements can be difficult when using a pressure regulator or a control valve. For those of you who are not familiar the concept of turndown ratio it is simply the ratio between the maximum flow requirements and the minimum flow requirements for an application.
Pressure regulators tend to have very large turndown capabilities and depending on the type of regulator and the application the turndown can be appear almost infinite. However, there are applications where a single control device cannot meet the turndown requirements. When a pressure regulator or control valve reaches its turndown limit, the result can be instability or even excessive wear on internal components leading to a lower mean time between repair (MTBR).
A common solution for applications with high turndown ratios is the installation of a minimum flow regulator. The minimum flow regulator is installed in parallel with a larger pressure regulator or control valve and sized to handle the low flow process conditions. When using two regulators in parallel, the minimum flow regulator will have a higher setpoint than the larger regulator and as demand increases, the downstream pressure will droop allowing the larger regulator to open. When in parallel with a control valve, the minimum flow regulator is typically set to achieve a minimum flow rate at the required pressure setpoint.
Minimum flow regulators can be found in a variety of applications including steam systems with winter and summer loading requirements, fired heater fuel gas systems that require a small amount of start-up fuel gas, or even tank blanketing applications where pump-in and pump-out rates are dramatically different.
If you have other applications where you use minimum flow regulators, please share below.
We have used similar setups in liquid pressure control service in the past. Liquid flow characteristics can actually negatively impact a valve's potential turndown. Below is a schematic that was proposed previously when working with a customer designing a down hole mining application requiring high pressure water control. It also incorporated an upstream wide-open monitor for over pressure protection purposes.
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