Thursday, 24 May 2012


These are found in impulse turbines to create the requisiste number of stages. They locate with a fixed row of blades and are sealed against the rotor shaft by a gland arrangement which must remian effective throughout the working range the diaphragm operates in.
They have a large surface area and so must have sufficient stregth to resist pressure drop across them without being excessively wide which would increase rotor length. Allowance must be made for rapid temperature fluctuations found during manouevring.
Teh take the form of a disc with a row of blades at the circumference and a hole at the centre for the rotor. A horizontal split allows for disassembly, rotation is prevent by a locking plate at the horizontal casing joint.

Methods of Diaphragm fixing

Alternate arrangement

Steam pressure holds the diaphragm plate hard against the downstream face.

Rotor Sealing

Loading on diaphragm = 207 to 280 NM/m2, deflection is approximately 2mm

Diaphragm material

In the high temperature regions typically Molybdenum-vanadium steel all parts. More generally a low carbon steel for the nozzle division plates and spacer bands, mild steel for rest. In the low temperature region cast iron diaphragms may be used. Alternatelychromium or Nickel alloy steel may be used

Construction of Diaphragm Nozzles (all riveted attachment)

Diaphragm is a loose fit in the slot in the casing to allow for expansion.

Construction of Diaphragm Nozzles (partial welded attachment)

The nozzle is assembled in batches by pushing the tenon of the blade throught the channel hole and riveting. A spacer is fitted and the whole tack welded. The blade batch is caulked into the casing. A small allowance is made for expansion.
Some sections on the first stages may be blanked where partial admission used.
Modern turbine designs have a curtis wheel first stage which absorbs a large portion of the energy in the steam. The exahust from this stage has a relatively high volume therefore all further stages are full admission.

Construction of Diaphragm Nozzles (welded attachment)

Modern diaphragms are all welded.Nozzle plates or guide vanves fit into slots in the inner and outer rim. The whole is welded to the centre body and perpipheral guide ring. Expansion is allowed for in the casing groove. The Nozzle blades or guide vanes are commonly made from stainless iron. The centre body from Chrome Molybdenum steel in higher temperature regions, mild steel for the lower.

Archaic design

Included for general interest .

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