Resolving unstable steam control at a dairy

The problem

In May 2023 a dairy asked us to look at two control valves on the steam lines feeding its evaporators. Under full demand from both evaporators at once, the plant could not hold a stable temperature. Operators called the effect “steam wobble”, and kept the process on spec by correcting the control settings by hand.

Evaporator steam is one of the harder duties in food and beverage processing: the load swings with production, and the valve has to hold temperature across the whole of it.

Each evaporator ran a DN40 control valve fitted with a Samson Type 3730-4 positioner. Both valves were already specified at the largest Kv available in that body size, so there was no trim change left to make.

What we specified

We read the positioners’ own diagnostic record straight off the valves during a site visit. The Type 3730-4 logs position and travel in normal operation, so the data came off without touching the control system or interrupting production. Samson’s technical team in Germany analysed the trend with us, and we took the finding back to the plant as a sizing recommendation. That combination, plant data plus the manufacturer’s engineering, is what our technical support exists to put together.

One valve, EV02VC06, showed the fault clearly. It sat near the top of its travel for much of the time and drove fully open whenever both evaporators pulled at once. A valve that runs out of travel at peak load has no authority left to control with, and that is what the plant felt as instability.

DN40 tops out at Kvs 25 and both valves were already there, so the capacity had to come from a larger body. Sizing is the whole job with control and isolation valves: a valve chosen on line size rather than on duty runs out of travel long before anyone calls it faulty.

Installed against specified valve

	Installed	Specified
Valve size	DN40	DN50
Kvs	25	40
Positioner	Type 3730-4	Type 3730-4

The larger bore gives each valve more flow capacity, so neither has to sit at the end of its travel to pass full steam demand.

The outcome

The dairy went ahead with the replacement. Since the DN50 valves were installed the plant holds temperature under full steam demand from both evaporators, and the control settings no longer need manual correction. The instability the site called “steam wobble” has not returned.

The valves looked correctly specified on paper. The fault showed up only in how they behaved under load, which is what the positioner data exposed.