The Velan Theory

The Velan Theory - What is Bimetal?


Bimetal is a composite material comprising two metals of largely differing expansivities.  When subjected to changes in temperature, this difference in expansion causes the bonded metals to bend.  This bimetal is specially designed to meet the specific requirements in the Velan steam trap.  It resists corrosion even in most severe alkaline conditions (12 pH)., and acids (2 pH).

Two bimetal materials have been specifically developed for use in Velan Steam Traps

  1. For saturated steam service and superheat up to 520 deg F a bimetal with the largest possible deflection and excellent corrosion resistance both in alkalis and acids.  Its temperature range is limited to 520 deg. F.
  2. For superheated steam service up to 1100 deg F a bimetal of high tensile strength unchanged at elevated temperatures.  Deflection of this bimetal does not increase above 600 deg F.  Consequently, no additional stresses are developed in the element when used on superheated steam.  It can safely withstand temperatures of 1100 deg F.

The Velan Theory - Velan Design vs. The Steam Curve


The steam line pressure acting on the valve surface develops a force keeping it open when condensate or air is in the trap.  This force is determined by orifice, valve diameter and steam line pressure. 

For a ½” diameter valve, for instance, a 150 psi line pressure develops a force of 30 lbs. on the valve.  When steam at saturated temperature enters, the bimetallic element pulls with a force of 35 lbs. and closes the trap.  To develop sufficient force against the maximum line pressure, for which a particular trap is designed, the element has to contain a sufficient number of bimetallic plates.   In order to use the same element for a large range of pressures and to get the same trapping and condensate discharge over the entire range, the bimetallic element must follow as closely as possible the saturated steam curve so that on the one hand, sufficient force is developed to close the valve when steam enters the trap, while on the other hand, the bimetallic pulling force has to decrease fast enough for condensate drainage.

A bimetallic element not subdivided in sections would have a straight line characteristic and the trap would operate only on large temperature differentials.

In the Velan steam trap, a patented multi-segmented construction was developed so that the temperature responsive element follows the saturated steam curve as near as possible.  Accordingly, the individual segments are mounted spaced apart on a holder and react at predetermined pressure and temperature. 

Relations between the bimetallic force and line pressure acting on the valve, are shown for the different and most common pressure ranges of Velan steam traps.

The individual segments of the bimetallic element act consecutively at predetermined temperatures and pressures, adding more pulling force when pressure increases to close trap.  At low pressure, only the first segment is active.  This is a most satisfactory solution to a steam trapping problem.  An ideal trap must recognize not only steam (being a gas) from water (being a liquid) but at the same time, react to the temperature of steam, to be able to discharge flash or wet steam below saturated temperature and also close tight on superheated steam.

On superheated steam, the Velan steam trap reacts instantly and tends to close the valve even tighter as the bimetallic element develops additional closing force.  We are proud to claim that this unique feature was introduced for the first time in the Velan steam trap.