As large swings in power demand occur, the need for plant cycling grows.

However, within a cycling plant's steam and water program, the need to obtain representative samples to control temperature, pressure and flow rate remains.

Flow-rate in particular sees considerable impact during the course of 24 hours. System pressure goes from a low load (~25%) at night to 100% during the day. This extreme fluctuation exposes the need for control.  

The primary control of sample pressure and flow rate in many power plants is the Sentry® VREL® control valve, which is a valve specifically designed to solve unique high-pressure sampling issues. The Sentry VREL is the industry standard for samples greater than 500 psig (34.5 barg) according to the Annual Book of ASTM Standards, ASTM Designation D3370.

The Sentry VREL consists of two stainless steel tubes joined to a large tube or barrel. A tapered rod assembly is inserted into precision holes in the barrel. The high pressure fluid enters, flows past one rod, turns and flows past the other rod, and then out. Pressure drop is a function of the length of the rods inserted into the barrel. The rod position is controlled externally by turning the knob, and can be adjusted while sample is flowing. If blockage occurs, the rods can be retracted so system pressure can blow the solid matter through.

Inconsistent sample flow in a cycling plant

With some power plants losing internal expertise through employee retirement, and to stay cost-effective, there are fewer and fewer operators available to adjust the Sentry VREL daily as needed.

How does this affect daily operation? Depending on the conditions, one of two scenarios have been witnessed in the field:

• First, the Sentry VREL is adjusted to the proper flow rate at 100% of plant load. However, when the plant cycles low enough, the sample flow is lost completely and the chemical analyzers send an alarm to the control room.
• Second, the Sentry VREL is adjusted properly at low load. However, when the plant reaches 100% of its load, the sample flow rate is way too high. It is not uncommon to see dangerously high-temperature water spraying into the sink and/or splashing all over the floor.

Additionally, a constantly varying sample flow rate can cause soluble contaminants to become dislodged from piping and plug the tubing further downstream in the system. This unpredictable flow rate also can cause unneeded wear on components, wasted cooling water, and samples that are misrepresentative.

How to manage flow for representative samples

Fortunately, there is an automated solution for obtaining representative samples within a cycling plant. The Sentry AutoVREL flow controller is designed for sample conditioning automation to meet the requirements of representative sampling, maintaining a constant flow rate with varying inlet conditions.

A flowmeter installed in the sample tubing provides automatic feedback to the Sentry AutoVREL flow controller, which in turn operates a motorized VREL unit. Setting the sample flow rate is as easy as adjusting a household thermostat – the operator just pushes the up or down arrows.

The Sentry AutoVREL flow controller is available as an option on all Sentry steam and water sampling systems, and also may be retrofitted on systems currently in operation. See an overview of operation in the video below.