When a steam generator level controller is obsolete, you usually do not need to replace the probe. In a quotation for the ammonia stripping plants at a landfill, the old capacitance probe stayed compatible with the new pre-amplifier and controller, so only those two parts were replaced, and the new controller was supplied with an adaptor to fit the old cut-out in the panel door.
The challenge: three parts, all superseded
The level of water in a steam generator is controlled by three components working together:
- A capacitance probe in the kettle, which measures the water level.
- A pre-amplifier on the head of the probe, which conditions the signal and sends it to the panel.
- A level controller in the control panel, which uses the signal to control the feed water inlet valve or the speed of the water pump, depending on the line, so as to keep the level steady.
On all four lines at the site, every one of these had been superseded by a newer model. If the level control fails, the steam generator can run dry or flood, so a failed controller cannot simply be left.
The approach: replace the electronics, keep the probe
What the probe tells you
A capacitance probe is a passive device in the vessel. It measures the water level by the electrical capacitance between the probe and the water. The supplier confirmed that the old probe remains compatible with the new pre-amplifier and controller, so it need not be replaced unless damaged. That avoids opening the pressure vessel.
Why the pre-amplifier and controller go together
If a new pre-amplifier is required then a new panel level controller is required as well. The new parts are a matched pair, and the old controller cannot read the new signal. In a related quotation at another site, the change also meant altering the controller’s input from a voltage signal to a milliamp signal, which is a configuration to check at commissioning.
Making it fit the panel
The replacement controller is designed to sit on a DIN rail inside the panel, but the old one was mounted in the panel door. The new controller comes with an adaptor, so it can mount in the same space in the door. That avoids cutting new holes in the panel and rewiring.
Why it works
The probe sits in the pressure vessel and is the hardest part to reach. Controllers and pre-amplifiers are small and easy to swap. By keeping the compatible probe and replacing the electronics, the work stays outside the pressure boundary and downtime is short. Delivery was two to three weeks to site.
What to consider for your own project
- Ask whether the sensor can stay. Many sensors outlast their electronics.
- Replace matched parts together. A new pre-amplifier needs a new controller.
- Check the signal type. Voltage and milliamp inputs are not interchangeable.
- Check the panel cut-out. An adaptor can save modifications to the door.
- Keep a spare. A level control is a single point of failure for a steam generator.
For the wider picture on instrument and drive replacement, see our post on replacing obsolete instrumentation and controls. The Health and Safety Executive’s page on pressure systems covers the duties that apply to steam equipment in the UK.
FAQ
What does a capacitance level probe do?
It senses the level of liquid by the change in electrical capacitance as the liquid covers more or less of the probe.
Do I have to replace the probe with the controller?
Not necessarily. Here the old probe worked with the new electronics.
Why does the controller control a valve or a pump?
Some lines hold the level by throttling a feed water valve, others by changing the speed of the feed pump.
How long does the replacement take to arrive?
About two to three weeks in this case.
Is your level control obsolete?
Send us the model numbers and we will check for a compatible replacement.