The Hidden Restriction
A PTFE heat exchanger loses 30% of its heat output. The steam pressure at the plant header is normal. The control valve is fully open. The heat exchanger tubes are clean. Yet the flow is restricted. The cause is a steam supply isolation valve-installed in the steam line between the header and the control valve-that is partially closed.
The valve may have been left partially closed after a maintenance shutdown, or its position indicator may be incorrectly showing "open" when the valve is actually throttled. The restriction reduces the steam flow and the heat output. The diagnosis requires verifying the actual valve position and the pressure drop across the valve.
The Diagnostic Signs
A partially closed supply valve produces a specific set of symptoms that distinguish it from other causes of reduced heat output:
Pressure drop across the valve: A partially closed valve creates a pressure drop. The steam pressure upstream of the valve is normal; the pressure downstream is reduced. A pressure gauge installed on either side of the valve confirms the pressure difference.
Reduced steam flow at full control valve opening: The control valve is fully open but the steam flow is below the expected value. The flow meter, if present, shows the low flow.
Normal upstream pressure with low downstream pressure: This is the key distinction from a boiler pressure problem-a boiler issue reduces the pressure everywhere; a valve restriction reduces the pressure only downstream of the valve.
| Symptom | Partially Closed Supply Valve | Boiler Pressure Problem | Fouled Heat Exchanger |
|---|---|---|---|
| Upstream pressure | Normal | Low | Normal |
| Downstream pressure | Low | Low | Normal |
| Control valve position | Fully open | Fully open | Fully open |
| Heat output | Low | Low | Low |
| Pressure drop at supply valve | High | Low | Low |
The Confirmation Procedure
The confirmation is a simple two-gauge test. Pressure gauges are installed on the steam supply line-one upstream of the supply isolation valve, one downstream between the isolation valve and the control valve. The readings are compared.
If the upstream gauge reads the normal header pressure (for example, 6 barg) and the downstream gauge reads a reduced pressure (for example, 2 barg), the supply valve is restricting the flow. The valve should be fully opened, and the pressure should equalize.
If both gauges read the same reduced pressure, the problem is upstream-the boiler, the header, or another restriction in the supply line.
If both gauges read the normal pressure but the heat output is still low, the problem is in the heat exchanger itself-fouling, flooding, or another internal issue.
The Valve Position Verification
The valve position should be verified manually. For a rising-stem gate valve or globe valve, the stem position indicates the opening. For a quarter-turn ball valve or butterfly valve, the handle orientation is the indicator. The position indicator may be faulty-a manual check of the actual stem or handle position is more reliable.
After the valve is confirmed fully open, the downstream pressure should recover to the upstream value. The heat output should recover to the design value. The recovery confirms that the valve was the restriction.
Summary
A partially closed steam supply valve restricting flow to a PTFE heat exchanger is identified by the pressure drop across the valve-normal upstream pressure with low downstream pressure. The two-gauge test confirms the diagnosis and distinguishes it from boiler pressure problems and internal fouling. The valve is manually verified and fully opened. The heat output recovers. The diagnosis prevents unnecessary maintenance on the heat exchanger itself.
Engineering support for steam system diagnostics is available upon submission of pressure readings, valve configuration, and observed performance loss.
