Snifting, Drifting, Bypass and Herdner Valves

Snifting, Drifting, Bypass and Herdner Valves

Snifting, Bypass and Drifting Valves were commonly used as a means of preventing negative pressures occurring in the cylinders of a locomotive during “drifting” or free-wheeling.  If a negative pressure does occur, it will immediately result in combustion gases and solid particulates being sucked down from the smokebox through the blast pipe, causing serious abrasion damage to the rubbing surfaces of both pistons and valves.

  • Snifting valves are simple one-way “check-valves” that allow clean air to be sucked into the cylinder in the event of negative pressure arising. These were commonly mounted on the side of the locomotive’s smokebox from where clean air could be drawn, thereby causing (undesirable) cooling of the cylinders.  Furthermore, admitting air into the steamchests and cylinders can result in oxydation of lubricants and the formation of deleterious gums, as described on the Tribology page of this website. Snifting valves can also leak, with obvious detremental effects. For further information see
  • By-pass valves simply allow steam (or air) to pass from one end of the cylinder with the other through a linking duct or chamber.  They are automatically closed by the admission of live steam to the steam chest and opened when the steam supply is shut off.  The main problem with use of these valves was steam leakage.  For diagram, see
  • Drifting valves differ from Snifting and By-pass valves, in that they admit a small quantity of live steam into the cylinder during drifting in order to prevent the creation of a vacuum in the cylinder.  Drifting valves are often incorporated into the design of the regulator.

Some railways (for instance the South African Railways) incorporated all three of these anti-vacuum valves into their locomotives, and encouraged their drivers (“engineers” in the US) to set their machines to drift in full forward gear, thereby relying on these devices to prevent any admission of gases from the smokebox.

In modifying SAR 2644 and 3450 “The Red Devil” Wardale stripped all three devices off both locomotives.

Wardale took sample indicator diagrams when running using three methods of drifting:

  1. valves in mid-gear; throttle shut;
  2. valves in full-gear; throttle cracked open.
  3. valves in mid-gear; throttle cracked open.

In case 1, he detected a momentary vacuum in the cylinder; in case 2, the steam flow became far too high; case 3 was found to be satisfactory, and was therefore recommended as the operating regume to be adopted for both locomotives.

See also Drifting Techniques page.

Herdner Valves are simple non-return valves that are fitted to the mid-points of a locomotive’s cylinders.  When opened, they admit steam directly from the steamchest into the cylinder, the steam flow being continuous regardless of the piston valve position.  As the piston moves from the end to the mid-point of the cylinder, steam admitted through the Herdner valve escapes directly to exhaust.  As the piston moves past the centre position, steam from the Herdner valve applies pressure to the piston until the valve reaches the point of release when the steam once again escapes to exhaust.

Steam is therefore wasted while the locomotive starts its train, but the starting cut-off is effectively increased to around 90% (as measured by the position of the piston at the point of release).  Once the train is started, the Herdner valve can be closed and steam flow to the cylinders is controlled normally – i.e. by the piston valves.

The photos below, taken from Wardale’s book “The Red Devil and Other Tales from the Age of Steam” show a Herdner starting valve mounted on the side of the cylinder of The Red Devil itself.