This article highlights various aspects of a new gas turbine shaft seal called the pressure activated leaf seal (PALS). The paper in detail discusses the architecture and working of a gas turbine seal. PALS is designed to use changing pressure drop forces across the seal to eliminate rub. The seal elements stay clear of the rotor seal surface during start-up and shutdown transients, and subsequently close to a small, non-contacting, steady-state running clearance. During start-up or shutdown, when the axial pressure difference across PALS is small, the leaves are in a relaxed open position, providing a general clearance gap for possible rotor seal surface eccentricities. At operating speeds, the resulting axial pressure difference causes the leaf element to elastically deflect and close, reaching the design clearance when they contact the support member. The test results show that the PALS concept provides for a potentially viable, robust, low leakage seal for gas turbine applications.
You may recall the words of Lewis Carroll's Alice in Wonderland walrus: “The time has come, to talk of many things: Of shoes - and ships - and sealing wax In a metaphorical sense, the design and operation of gas turbines depend on lots of “sealing wax.” Seals abound in both aviation and non-aviation gas turbines.
At our TURBO EXPO ’14 in Dusseldorf, there was an interesting paper  given on a new gas turbine shaft seal, called the pressure activated leafseal and appropriately acronymed as PALS. Clay Grondahl, its inventor and coauthor of the paper, alerted me to its unique features and advantages. The PALS assembly is shown in Fig. 1, but before we explain its operation, let us briefly review gas turbine seals in general.
The Seal Story
The extensive review of current seal technology by Chupp, et al., shows that most gas turbine seals don’t seal - at least not completely - but reduce undesirable flows. Seal locations are primarily in engine mainshaft areas and in gas path locations. Seals are used to prevent oil leakage from engine bearings, to reduce tip leakage in rotor blading, and in general to reduce air leakage between high and low pressure components, both rotating and stationary.
How important are seals? By and large, an engine can’t have sustained successful operation without seals. In addition, consider a simple economic example. Take the case of a large electric power gas turbine, whose annual fuel cost might be on the order of $100 million. A seal improvement in one location might increase engine efficiency by 0.1%, which is seemingly not large. However, that would result in a fuel savings of about $100,000 annually, and about $1 million for a fleet of 10 such machines.
Labyrinth Annular Gas Seals
Let us focus on the intended application of PALS, as an air- to-air gas turbine rotor seal between stationary and rotating parts. This application is critical for both power generation and aviation gas turbines performance, where the goal is to minimize leakage between high and low pressure turbine sections (and the intermediate section, in the case of a three-spool engine).
Consider the labyrinth seal shown in Fig. 2A (taken from), fixed on a turbine case, separating high and low pressure regions, and suspended to provide a seal clearance to a large diameter rotor seal surface. Labyrinth seals, named for the maze-like structure in Greek mythology where the Cretan Minotaur dwelled, are widely used in turbomachinery, going back to steam turbine inventor Charles Parsons’ machines in the early 1900s. There are many newer, more advanced seals (e.g. brush seals) which are treated in , but labyrinth seals are still used extensively in turbomachinery.
The annular labyrinth seal in Fig. 2A has teeth (like a course comb) that form successive cavities. As leakage air flow passes from high pressure under each tooth, it expands in an irreversible process into the next cavity, so that a series of thermodynamic throttling processes occur, dropping its pressure, as it approaches the low pressure region. A tighter clearance gap makes the sealing throttling process more effective.
However, during actual engine operation the clearance gap can vary, as shown by the sketch in Fig. 2B. The rotor seal surface diameter increases in response to shaft speed and rising gas path temperatures. On startup and shutdown, the clearance gap can close down due to the changing engine conditions. Also, the center of rotation of the rotor seal surface may shift as shaft bearing lubrication varies or shaft vibrations occur. Asymmetric thermal growth of support structures and sudden load changes can lead to circumferential clearance gap variation. Inspection of used seals reveals that these effects result in tooth rub which wears away the teeth or deforms them. In some cases an abradable coating on the rotor surface is worn away. In all cases the clearance gap is irreversibly enlarged, leading to more leakage and reduced efficiency.
PALS to the Rescue
PALS, shown in Fig. 1, is designed to use changing pressure drop forces across the seal to eliminate rub. The seal elements stay clear of the rotor seal surface during startup and shutdown transients, and subsequently close to a small, non-contacting, steady state running clearance.
The seal elements, or leaf seals, are formed from the two layers of circumferentially slotted, wear-resistant alloy shim stock, as shown in Fig. 1. The two slotted layers are imbricative, i.e. arranged a bit like overlapping shingles on a roof, such that each slot is covered by an upper or a lower leaf.
During startup or shutdown, when the axial pressure difference across PALS is small, the leaves are in a relaxed open position, providing a generous clearance gap for possible rotor seal surface eccentricities. At operating speeds, the resulting axial pressure difference causes the leaf element to elastically deflect and close, reaching the design clearance when they contact the support membershown in Fig. 1.
Fig. 3  gives a good overall picture of a PALS operation cycle. As reported in , test results show that the PALS concept provides for a potentially viable, robust, low leakage seal for gas turbine applications.
You may ask just how reliable is a leaf seal? As you read this, consider the multiple leaf seals in your own heart. Your aortic valve has three such leaflets, that open and close 50-70 times a minute - hopefully, for many years. Their operation is governed by the same fluid dynamic forces as those in PALS.