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Production Process Insights

Problematic Sample Flow? Here's How to Solve It

Posted by Jason Thomas on 8/24/20 8:00 AM

Solving Problematic Sample Flow

Sample flow is a primary factor that affects the results from a steam and water analysis system (SWAS) in a cycling power plant. When sample flow is inconsistent, so are analyzer measurements – which can lead to catastrophic equipment failures. Controlling flow in a SWAS requires the appropriate equipment to eliminate analysis compensation and sources of error in measurements to improve chemical feed accuracy and cost savings.

detecting problematic flow

Many SWAS in cycling plants fail to consistently manage online analyzer flows during startup or low load, and they fail to properly mitigate high iron transport during startup or load changes. System pressure goes from a low load (~25%) at night to 100% during the day. This extreme fluctuation exposes the need for control. 

This disruption in flow can affect daily operation. Sometimes a flow control valve 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.

A flow control valve could also be adjusted properly at low load. However, when the plant reaches 100% of its load, the sample flow rate is way too high. It’s not uncommon to see dangerously high-temperature water spraying into the sink and/or splashing on 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 and wasted cooling water.

Automating CONTINUOUS sample flow

When utility sample system operators experience problematic sample flows, they need to automate the flow to ensure reliable and accurate sampling results.

Frequent cycling or sliding pressures require lab technicians to manually adjust equipment to maintain sample velocity, which can be a challenge with limited staff and resources. By using a solution that automates the flow, the operator only needs to set the desired sample flow and the system automatically adjusts to changes in the sample pressure, decreasing the amount of necessary resources.

The best solution is an automated solution for obtaining representative samples within a cycling plant. An automatic flow control valve 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 flow control valve, which in turn operates a motorized unit. Setting the sample flow rate is as easy as adjusting a household thermostat – the operator just pushes the up or down arrows.

Addressing problematic sample flows

  • Make sure you have the right components in the right locations on the sample conditioning racks and panels.
  • Ensure cooling water supply valves are open and velocities are set correctly to design spec. They typically shouldn’t exceed 120-140GPM.
  • Check if isolation valves/HP blowdown valves are leaking. These should be open or closed, but should never be throttled.
  • Assess if there’s any indication that the system is losing steam from leaking blowdown valves.
  • Check coolers for hot spots or rattling noises. Leaking coolers can cause enough loss of flow to impact critical analyzer readings. Cooler plastic labels will appear burnt if exposed to excessive temperatures. Implement thermal image coolers if available. If not, touch the cooler shell from the bottom up using the back of your hand.
  • Make sure that pressure reducing valves (PRVs), such as needle valves and variable pressure-reducing elements (VRELs), are open and properly adjusted.
  • Install VRELs on any high-pressure sample lines (500psi or greater). Proper pressure reduction can help increase the efficiency of sample cooling upstream, thus improving overall sample conditioning for more accurate analysis. If you have lower pressure samples, test the needle valves.
  • Assure proper sample velocity. Use total flow indicators or use a stop watch and volumetric glassware to verify your total and individual sample flow rates in cc/min.
  • Install Back Pressure Regulating Valves (BPRVs). Regularly inspect BPRVs for visible leaks from weep holes. High temperatures can damage diaphragms in these devices, resulting in sample leaks. Forward versus back pressure regulators provide different functions. A BPRV provides protection of delicate equipment while supplying steady grab sample flow at the sink (available in varying pressures from 5-60 psi).
  • Sequence sample points and drain lines, and use flow rate control valves (FICVs) on the sample lines. Check the sequencer manifold drain lines for excessive flow that can waste needed sample flow. Adjust the tension on the manifold drain lines or install FICV on drain lines to quantify waste when not sequencing each sample point.
  • Use demin flush or other valves, with or without automation.
  • Place dedicated FICVs ahead of any analyzers without flow control gauges. Constant head flow control devices can temper any interruptions in flow due to drops in pressure or plant cycling. While excellent in some applications, these devices can send more process water to drain than you’d like. 

Proper design of sample systems puts critical components in the most ideal locations. Sentry Equipment has extensive field experience applying flow control valves in SWAS solutions around the world to solve sample flow challenges and give sampling systems more flexibility to handle changing conditions.

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Topics: Power, Steam & Water

Written by Jason Thomas