Steam Turbine Diaphragm Repair: Options, Upgrades and When to Replace

A steam turbine diaphragm is the stationary ring of nozzles that turns steam pressure into a fast jet and aims it at the next row of moving blades. Steam turbine diaphragm repair works when the damage is confined to the partitions and seals: eroded trailing edges rebuilt by welding and machining, damaged partitions replaced, seals renewed. A diaphragm should be replaced when the structure has deflected, the welds that hold it together have cracked, or the original design can no longer carry what the plant now asks of it.

The decision depends on measurement, not appearance. This article covers how a diaphragm is assessed, what each repair involves, which upgrades are worth considering, and when a replacement made by reverse engineering beats another repair. Our guide to steam turbine blade damage covers the damage mechanisms by location in the steam path.

What a Steam Turbine Diaphragm Does

A diaphragm is a ring split into two halves at the horizontal joint, seated in the casing between rotor wheels. Its partitions, also called vanes or nozzles, form passages that speed the steam up and turn it towards the next row of blades. In the control stage the same job is done by a nozzle ring or nozzle block, which is why "steam turbine nozzle repair" and "diaphragm repair" describe one family of work.

A 2013 paper by Siemens and Saudi Aramco engineers, presented at the Second Middle East Turbomachinery Symposium, lists what the diaphragm has to do. It turns steam pressure into velocity, directs the jet into the rotating blades, sets the pressure drop of the stage, keeps deflection low enough to prevent rubs, and houses the stage seals. It also divides the casing into compartments, so a failure in one stage does less damage.

Loading explains why diaphragms fail. In an impulse stage almost all of the pressure drop happens across the stationary row, so the diaphragm takes the pressure load, and the paper says the load is heaviest in the first few stages. The outer ring sits in the casing while the inner ring hangs free. Welding is the most common way to build one. A typical welded diaphragm has stainless vanes set through two thin spacing strips, joined to inner and outer rings of carbon steel, carbon-moly or chrome-moly, then stress relieved and machined.

How Diaphragms Get Damaged

The blade damage article covers erosion, foreign objects and deposits along the whole steam path. These are the mechanisms that belong to the diaphragm itself.

Trailing edge erosion. Solid particles in the hot stages and water droplets in the wet stages thin the partition trailing edge. That edge sets the exit angle and the opening between partitions. A General Electric patent calls it the critical component of the partition.

Cracking. A temperature difference between the upper and lower halves can set up forces strong enough to crack partitions and free them. Water or condensate getting into the machine is the usual cause.

Dishing. The pressure load pushes the diaphragm downstream, and at temperature a weak material or a shallow structural weld lets it creep that way until it rubs the next stage. At the two Saudi gas plants in the symposium paper, deflection grew as production rose. Between 2005 and 2009 four steam turbines suffered diaphragm and rotor failures from axial rubs, and one stage 4 diaphragm disintegrated completely.

Structural weld failure. If the welds tying the partitions to the rings fail, the diaphragm can collapse into the downstream blades. Poor tie-in, poor weld application and erosion that undercuts the weld are the usual routes.

Loose partitions at the joint. In another Aramco turbine, with an undamaged rotor, the vanes at the horizontal joint of the stage 5 diaphragm could be moved by hand. Stresses are highest at the joint on any diaphragm, so look there first.

Seal wear. Tip and shaft seals sit in the diaphragm. When the rotor moves further than the clearance, at a critical speed or in a sharp load change, rigid seals rub. The fins wear and leakage rises. In some cases a seal cuts a groove in the shroud, which means replacing the whole row of blades.

Damage mechanisms that belong to the diaphragm itself and what each one does. Source: Siemens and Saudi Aramco, Middle East Turbomachinery Symposium, 2013; General Electric patent US6754955; MIMAH engineering practice.

How a Diaphragm Is Assessed Before Anyone Decides

Record the as-found condition and take any deposit samples, then clean the diaphragm and measure it. Five checks give the evidence for the repair decision.

Visual survey. Look for rub marks on the downstream face, loose or cracked partitions at the horizontal joint, cracks at the welds and the condition of the trailing edges. Photograph with a scale in frame.

Non-destructive testing. Penetrant or magnetic particle testing on surfaces and welds, and ultrasonic testing where weld depth matters. Name the method and the acceptance criteria in the plan before the casing comes off. EPRI's 2012 summary of turbine repair methods describes the limits of each method.

Deflection. Measure the axial position of the diaphragm face at several points around the ring and at the joint, and compare it with the as-built record and the last survey. The rate of change matters as much as the value. Judge distortion against the manufacturer's limit and the clearance to the rotor.

Throat openings. Measure every opening at its narrowest point. The average and the spread both count.

Material and weld verification. Verify the material before any welding. A repair also needs the construction type and the axial depth of the original structural weld.

Why Throat Area Matters for Flow and Stage Loading

The throat is the narrowest part of the passage between two partitions. Its total area, across all the partitions in the ring, decides how much steam the stage passes at a given pressure.

A first-order estimate shows the size of the effect. For a choked row, where the steam reaches sonic speed in the throat, NASA Glenn's mass flow choking page gives the flow as proportional to the throat area times the total pressure ahead of it, divided by the square root of the total temperature. That relation is written for an ideal gas, so for steam it is an approximation.

Take a hypothetical stage with a design throat opening of 12.0 mm. Trailing edge erosion has opened the measured average to 12.6 mm. With the same number of partitions and the same height, the throat area has grown by 12.6 / 12.0 = 1.05, which is 5 percent.

If the row is choked, then at the same steam flow and temperature the pressure ahead of it must fall to 1 / 1.05 = 0.952 of its old value, a drop of 4.8 percent. On a stage that normally runs at 2.50 MPa, that is 2.50 / 1.05 = 2.38 MPa. Most rows in a turbine are not choked, and there the pressure behind the row props up the pressure ahead of it, so the fall is smaller. Read 4.8 percent as an upper bound for one row. What you can rely on is the direction: a stage pressure that falls at constant flow is an early sign that a throat has opened.

Stage loading shifts as well. The pressure drop between inlet and exhaust is fixed by the plant. A row with a bigger throat needs less pressure drop to pass the same flow, so the rows ahead of it must carry more. The moving blades behind the eroded row also receive steam at the wrong velocity and angle.

The spread of openings matters as much as the average. Each moving blade sweeps past every passage in turn, so uneven openings become an uneven push on the blades. The symposium paper ties thermal distortion of a diaphragm to lower efficiency and more stimulus on the rotating blades. A repair that restores the average but leaves the spread has fixed half the problem.

Why throat area matters: a first-order estimate for an invented choked stage, showing how a 5 percent larger throat lowers the pressure ahead of the row at the same flow. Source: NASA Glenn, Mass Flow Choking; MIMAH engineering practice.

Repair Options for a Damaged Diaphragm

Blend and dress. Small nicks and shallow damage are ground out to a smooth profile, within the manufacturer's limits, to remove stress raisers. Re-test afterwards to prove the indication has gone.

Weld build-up and profile restoration. This is the standard repair for eroded trailing edges. The General Electric patent describes the older method: grind off the damaged edge, weld new metal on, and contour both sides by hand. It says this could set the length of the whole outage. The method it claims over-builds the weld, then machines the profile with shaped electrodes by electrical discharge machining under a liquid dielectric. EPRI adds two cautions. Welding and grinding restores the original contour but typically does not make the nozzle more resistant to erosion, and the throat area has to be re-established after welding.

Partition replacement. When erosion or cracking goes beyond what weld build-up can restore, the damaged partitions or sections are cut out and new ones fitted. Weld heat is the risk. The symposium paper says sub-arc welding puts 8 to 10 times more heat into a diaphragm than electron beam welding, and the distortion that follows is what costs efficiency. Agree the distortion controls first, and re-measure every throat opening afterwards.

Dishing correction. A dished diaphragm can be straightened or re-machined, but the steel has already crept and the load has not gone away. We would not accept a shape correction on its own. Ask what stops the diaphragm moving again, and ask to see the stress analysis behind the answer.

Structural weld repair. This needs the construction type, the original weld depth and an understanding of how stress passes through the assembly. Post-weld heat treatment has to be chosen for each material.

Seal renewal. Seal strips are replaceable on most diaphragms. They can be straightened and sharpened at a major inspection, but we would usually replace them, which means planning the spares in advance.

Re-machining. After heavy welding the diaphragm is stress relieved and machined again, then checked for fit in the casing grooves and for axial position. A small change in position can create a rub.

Upgrades That Go Beyond Repair

Erosion resistant coatings. Boride and carbide coatings are the most widely used, according to a Siemens patent from 2010. It says crack-free boride coatings are close to impossible to produce on real parts, and that cracks significantly reduce fatigue strength. A patent is arguing for its own invention, so take that as a warning and not a verdict. Ask a coating supplier for fatigue evidence on your material, not only erosion test results.

Retractable and brush seals. The symposium paper describes seals that spring open during start-up and shut-down, so the fins do not rub while the rotor passes its critical speed, and close under steam pressure once load passes about 5 percent of rating. It reports that standard seals wear until their leakage is two to three times the new value, while brush seal leakage stays close to constant. The lead authors work for a turbine maker, so test that against your own leakage figures.

Stronger, redesigned diaphragms. The Aramco story is the useful case. Spare diaphragms of the original design were bought in the late 1990s. Later analysis showed both the original and a modified design lacked the strength for the plant's operating conditions. The finite element work is the part to study: the original vanes modelled with full-penetration electron beam welds still had high stresses and nearly the same deflection as the sub-arc original. Better welding would not have saved them. The answer was a drop-in diaphragm with new airfoils, made to fit the existing casing and rotor with no rework.

New nozzles with new blades. New vanes that have to work with original blades cannot be set at their best angle, the authors note, so they cannot deliver their full efficiency. They estimate a further 8 to 10 percent gain from pairing new diaphragms with modified rotors, and say the design is much more expensive. Aramco postponed it.

When to Replace, and Why a Copy of the Old Part May Be the Wrong Answer

Replace when the structure is the problem: collapsed or cracked structural welds, cracked rings, deflection that has caused rubs and cannot be machined out, or a diaphragm that has been repaired several times and keeps coming back. Replace when the part has become obsolete.

The Aramco case carries a warning for anyone reverse engineering a part. The paper puts the failures down to a mismatch between the original design and the new operating conditions, not to operating or maintenance mistakes. Copying the drawing would have reproduced a diaphragm already failing under the plant's present load. So before a copy is made, ask what the plant asks of the stage today and check the design against it. Our steam turbine overhaul cost article covers the lead-time and contract questions that follow.

Reverse engineering makes sense when the manufacturer's part is unavailable or slow. The sequence is: measure the worn part and its seat in the casing, verify the material, check stress and deflection against current loads, fabricate and stress relieve, machine, inspect every throat opening, and prove the fit in the casing. MIMAH's engineering team does reverse engineering and local manufacture of critical spares, and treats the old drawing as a starting point.

In the Shop or on Site, and Who Does the Work

Weld repairs, heat treatment, machining and electrical discharge profiling belong in a shop with the equipment and process control. Seal renewal and light blending can often be done on site. Specialists come from several places: the manufacturer's service organisation, independent repair shops, non-destructive testing firms and field machining crews.

Whoever it is, ask which welding procedure and heat treatment they will use, how they will control distortion, how the material is verified before welding, how they will measure throat openings afterwards and to what tolerance, and what you receive at handover. Ask for the answers as documents. Our steam turbine repair guide lists wider contractor questions, and the inspection checklist shows where diaphragm measurements sit in an outage.

Repair, Upgrade or Replace: A Decision Guide

Repair when the damage is confined to trailing edges, partitions or seals, the structure measures within limits and the rate of change between surveys is slow.

Upgrade when the plant's duty has changed, when a stage keeps wearing in the same place, or when seal leakage is the main loss. Price the gain against the cost, as Aramco did.

Replace when the structure has failed or is moving, when repeated repairs no longer hold, or when the part is obsolete. If you replace, make the new part for the load the plant has now.

Measure the diaphragm first, then choose. Water induction, the usual cause of thermal-mismatch cracking, is covered in our guide to common steam turbine failures.

When to repair, upgrade or replace a steam turbine diaphragm. Source: MIMAH engineering practice.

MIMAH's industrial engineering team is independent of any turbine manufacturer, and has an ongoing rehabilitation of a 14 MW steam turbine in Nigeria. If you have a diaphragm survey to review, a repair to scope or a part with no supplier, get in touch.