Piston Valve Design

Various other pages of this website are devoted to specific aspects of piston valve design including:

This page serves to summarize the overall principles covering valve design as defined by Porta including linked references to the above pages. It also includes the following sections:

Note: All discussions about valves on this website relate to piston valves which were favoured over other types by both Porta and Wardale. A separate series of pages are devoted to an extensive discussion by Wardale on the relative merits of Piston Valves vs. Poppet Valves.

Porta’s Principles relating to Valve Design

Porta’s principles relating to valve design focus on two disparate criteria:

  1. Limiting energy (pressure) losses as far as possible;
  2. Minimising friction and wear.

In Wardale’s design of the 5AT, a third criterion was added – viz: keeping the masses of the reciprocating parts (valves, rings, valve rod etc) as small as possible so as to minimise interial forces.

Taking each of the first two criteria in turn:

Limiting energy (pressure) losses – This can be achieved by:

  • Using the maximum possible diameter of valve to give the largest possible port area.  In the case of the 5AT, Wardale achieved this requirement through the use of twin Φ150mm valves driven through a rocking shaft which provided the same port area as a single Φ350mm valve. The twin valves served to reduce substantially both the reciprocating masses and the clearance volume which would have been excessive had a Φ350mm valve been mounted above a Φ450mm cylinder.
  • Reducing the clearance volume as far as possible (without excessive restriction of the steam passages).
  • Streamlining the valve heads and ports to minimise wire-drawing pressure drops and triangular losses – see the Internal streamlining page;
  • Optimising valve gear geometry, valve travel and valve events – including lap and lead – to best suit the intended duties of the locomotive.
  • Minimise steam leakage, especially past valve and piston rings by using multiple narrow rings with small ring gaps, and aligning ring gaps along bottom of valve heads.

Minimising Friction and Wear – This can be achieved by:

  • Applying lubricants directly to the rubbing surfaces rather than mixing it with steam being fed into the steamchest. In the case of piston valves, it is important that lubricant be applied “between the rings” – see Lubrication page;
  • Limiting the temperatures of rubbing surfaces by
    • use of heavy bridge bars to more rapidly conduct heat from the admission side of the liner to the exhaust side;
    • use of multiple narrow rings to more efficiently transport heat from the hot (admission) side of the liner to the (cooler) exhaust side;
    • use of a diffuser mounted outside the valve head on the exhaust end to direct cooling exhaust steam over the hotter surfaces at the admission end of the valve head;
    • where very high temperature superheated steam is used, provided saturated steam cooling of the rubbing surface on the admission side of the valve liner.
    • use of bronze rings and crown facings to polish liner surfaces and reduce wear.

Porta incorporated most or all of these principles into the diagram that he sketched for inclusion in his proposal for the A1 Tornado, as below. The sketch is probably unique in that it shows how a valve may be enlarged without altering the outside dimension (or appearance) of the valve casing.

Fig 34: Fabricated Cylinder End
as proposed by Porta for A1 Tornado

The legend from Porta’s paper explaining each itemised component, is as follows:

“The outline adheres to the original one [for the A1]. The liner (1) is pressed by the cover (2) secured by SAE 4340 studs (3). Dimension ‘a’ is kept to a minimum, thus for a given ‘b’, the valve diameter can be maximized and the clearance volume made as small as possible.

(4) is a diffuser also performing a cooling action for the (valve) crown (5). (6) is a sealed insulation; (7) saturated steam liner cooling grooves;

(8) and (9) are hollow, welded-on BRIMs filling the clearance spaces. The space (10) is filled with glass wool mattress. (11) is a dummy protrusion for appearance.

The valve liner is 30% larger than the original; valve lap is 55mm (against 38), and the flow coefficient is 0.96 instead of ~0.60, all making ~2.5 times greater flow area at short cut-offs.

Hooks (12) are a trick to keep unchanged the valve chest cover diameter. The width of the steam passage (13) has been increased from 50.8 to ~60mm, thus resulting in a ~300cm2 area. Were it not for respecting the outermost dimension of the cylinder, it would have been desirable to increase it to 400cm2 so as to have 1:4.6 ratio with piston area: this affects the performance at very high speeds (see Section 20.2).

Clearance volume is estimated as 8% of cylinder volume, a rather small figure due to brims (8) and (9).

Liner(s) (7) are of the through type from end to end (in three parts) thus avoiding “torturing” valve rings at the conical entrances. Radiuses (14) through (17) are important to have not a “vena contracta”. (18) is the position of the platform above the outer cylinder.

This basic design was first applied by the writer on prototype engine No 3477, FCGR, on Rio Turbio engines, on prototype No 4674 FCGB, and by Wardale in prototypes 2644 (19D) and 3450 (Class 26), SAR.”

Note: The trapezoidal ports are not the type that Chapelon recommended in which the ‘vertical’ faces are sloped in order to graduate the ‘release’ of exhaust steam so as to reduce firebed disturbance (see page 97 of Wardale’s book). In this case, it is the bridge bars that are inclined in order to spread the area of contact with the valve rings thereby reducing their wear rate.

Multiple Narrow Piston and Valve Rings

Porta recommended the use of multiple narrow valve and piston rings, the word “multiple” meaning “as many as possible within the length of the valve of piston crown”. In his Fundamental Design Calculations for the 5AT, Wardale specified multiple narrow rings for the purpose of “minimising the sealing duty of any individual ring, reducing ring-liner friction and wear, and enabling steam tightness to be maintained for extended periods of service without inspections or maintenance”.

Porta recommeded the use of multiple narrow rings in several of his papers, describing their advantages in the following generalized terms:

  1. Narrow rings are more flexible and therefore provide better contact with the liner when thermal and other stresses cause distortions to the ring shape.
  2. Narrow rings can more readily pick up heat from the hot rubbing surface of the liner (on the admission side of the port) and transport it to the cooler exhaust side of the port, thereby assisting in the process of cooling the liner (and improving lubrication).
  3. Use of narrow rings allows more rings to be fitted on any given length of crown. If sufficient rings can be fitted to ensure steam-tightness, there may be space to add some bronze rings to help polish the rubbing surface of the liner and thus improve lubrication.
  4. Lower mass and therefore lower reciprocating inertial forces – low enough that steam pressure can prevent the outer ring on the admission side from touching a thin (streamlined) edge land.
  5. Ring-mounting stresses and distortions in narrow rings are less than in wide ones.
  6. The steam pressure differential across the ring set is distributed between the rings, thus the use of as many rings as possible reduces the steam pressure on each ring and thus leakage past each.

The illustration below (Plate No 31 from Wardale’s book) shows one of the valve heads from the Red Devil on which 12 rings were mounted. The left side is the admission edge with a very narrow streamlined land as per Porta’s recommendations. The exhaust edge land is much more robust because it has to carry inertial loads from the outer ring.

Wardale’s description of the image is as follows:

“Close-up view of a valve head showing the thin yet adequately robust admission edge land which allowed ring-control of steam admission and cut-off whilst giving a good steam flow coefficient at all port openings Also visible are the set screws for holding the ring-cuts at the botom of the valve when the assembly was mounted in the locomotive.”

Valve Liner Cooling using Saturated Steam

Where very high superheat temperatures are to be used, Porta recommended the use of saturated steam to cool the rubbing surfaces of valve liners. He illustrated the concept in the diagram (above) that he prepared for the A1 Project.

Wardale adopted the idea for his Red Devil and also for the 5AT. Below are two illustrations from Wardale’s book showing the grooves machined into the outer faces of one of the valve liners for the Red Devil. Below them is the sketch that Wardale prepared to illustrate the liner cooling system for the 5AT.

Plate 28. 26 Class No 3450: cooled through-type pearlitic cast iron valve liner

Plate 29. Detail of the valve liner cooling passages.

Diagram of Valve Liner cooling system
from 5AT Fundamental Design Calculations

Post Script: In his Fundamental Design Calculations for the 5AT, Wardale devoted 180 lines of calculation to the design of the cooled valve liners. He concluded the calculations with the following note:

“The principle of cooling by saturated steam is applied in these calculations to the valve liners, which are the hottest of the various engine rubbing surfaces and therefore in the greatest need of cooling. However the same principle can be applied to any other rubbing surfaces, such as the cylinder liners and piston rod / tail rod packings, should cooling of these items be shown by service experience to be necessary. Cooling using boiler feedwater under pressure, by piping some of the feed from the feedwater pump to cooling passages for the surfaces concerned then back to join the main feedwater flow upstream of the feedwater heater, is also a possibility (similar to water cooling of i.c. engines). This may be particularly suitable for piston rod & tail rod packings.”