View Full Version : HPTCC: High Pressure Turbine Clearance Control

12th Aug 2007, 12:01
CFM56 (737 and A320 at least) have High Pressure Turbine Clearance Control.

This adjusts the size of the cowling to perfectly match with turbine blades, if I understand correctly.

How does this work exactly?

More: MEL mentions High Pressure Turbine Clearance Control Timer(s).

What's that?

12th Aug 2007, 13:56
How does this work exactly?

Keep it simple,

Case coverings are thin and closer to the fire and get hot quicker than thick turbine disks. Therefore they expand quicker under the heat and open up blade tip clearances which reduce engine efficency.

The idea is to slow down the heating rate of the cases until the thick turbine rotor disks can catch up, so you meter some cooling air arround the outside of the case for a period of time to adjust the differences in themal growth.

13th Aug 2007, 17:30
I have the presentation from CFM, with the data related to the CFM56-5B (full FADEC) on the Airbus 320. It says the HPTCC uses HPC 4th and 9th stage bleed air. The graph shows that in cruise there is the highest airflow through the 4th Stage HPTCC air supply to the turbine casing. At full power there is the greatest amount of airflow from the 9th stage.

This is controlled by the ECU, which in turn uses the HMU to supply servo fuel to control the valve position.

There is also the following statement:

No air: Both butterfly valves closed: Failsafe position
9th Stage: 9th stage valve open only: Idle, takeoff, start.
Mixed: Both butterfly valves open: Climb.
4th stage: Only 4th stage open: Cruise.

By the way there is also low pressure turbine clearance control using fan air.

All the best.

13th Aug 2007, 18:01
My training manual says:

The high pressure turbine (HPT) clearance control system, uses high pressure compressor (HPC) bleed air to obtain maximum steady-state HPT performance and to minimize exhaust gas temperature (EGT) transient overshoot during rapid change of engine speed.
The system consists of:
● HPT Clearance Control Valve
● HPT Clearance Control Timer (on 56-3C1 and optional on 56-3B-2 engines)
● HPTCC Timer Lockout Solenoid Valve (on 56-3C1 and optional on 56-3B-2 engines)
● HPT Schroud Manifold Air Tubes

HPT Clearance Control Valve
The valve consists of two pistons operating two butterfly valves for the 5th and 9th stage bleeds. Valve operation is hydraulic using fuel from the main engine control.
After engine start and with the engine at ground idle power setting, the airflow to the HPT shroud is from the HPC 9th stage bleed. When the thrust lever is advanced or retarded to change the core engine speed (N2), the airflow is regulated to maintain optimum HPT shroud to blade tip clearance.
The airflow selection for various steady-state power settings is:
● Idle RPM....................9th Stage
● Take Off RPM............ 9th Stage
● Climbe RPM.............. 9th and 5th Stage
● Cruise RPM............... 5th Stage

HPT Clearance Control Timer
The High Pressure Turbine Clearance Control (HPTCC) timer controls the operation of the HPTCC valve during the first 182 seconds of engine operation if N2 RPM is 94% or greater during take off condition. The HPTCC solenoid valve prevents timer operation after take off through an air sensing relay.
The timer will be activated if:
● Aircraft on GRD and
● N2 RPM >94%

The normal schedule will be interrupted for 180 sec. The following cooling air schedule will be performed:
● 0 - 8 sec........... no air
● 8 - 152 sec........... 5th stage
● 152 - 182 sec........... 9th and 5th stage
● >182 sec............9th stage ( normal schedule )
With the shutdown of the engine ( RPM < 5% N2 ) the timer will reset automatically.

HPTCC Timer Lockout Solenoid Valve
Depending on the AIR/GRD switching the lockout solenoid valve activates the timer.
● On GRD............deenergized, the timer is activated.
● In AIR................energized, the timer is deactivated.

13th Aug 2007, 19:58
Hi IFixPlanes!

Haha I was in EDDF perhaps 4 days ago I think, memory shot, too much flying, goddam the cloud base was low, a proper 100 to go....minima and see the lights!!

It is interesting the 4th/5th stage difference. My A320 CFM doc simply states the following: The HPTCC controls the HPC 4th stage (Air) (-5A 5th stage).

Acronyms are a real annoyance (on a tangent):

HMU = FCU in old speak
FCU = MCP in Boeing speak etc.etc. doh!

13th Aug 2007, 20:13
Like everything else in life, designing engines involves compromises.

If the gaps between the turbine blade tips and their casings are too large, a lot of gas will leak around them instead of passing through the turbines. This will reduce the efficiency of the turbines. So efficiency is best when these gaps are small.

But turbine blade length does not remain constant. When running with high rpm and high temperatures such as during take-off, thermal expansion causes the blades to expand. So if the gaps are too small when the engine is built, the blades are likely to rub against the casing when running hot and fast.

The combined effects of high temperatures and high rpm also cause blade creep, which results in permanent increases in their length. The amount of creep increases with hours run, so the blades gradually become longer as engine age increases. If the gaps are too small when the engine is built, creep will cause the blades to rub against the casings. So for a long engine life without turbine tip rub, the gaps at the tips need to be reasonably large when the engine is built.

Active clearance control provides a useful compromise by shrinking the casings by the correct amount to match engine running conditions. This is achieved by blowing cool air onto the outside of the casings. This cools the casings, causing them to contract, thereby reducing tip clearances.

During take-off the blades are comparatively long, so very little contraction of the casing is required. So fairly hot air (from the 9th stage of the compressor for example) is used for the coolling process.

In the cruise the lower rpm and turbine temperatures mean that the blades are shorter and the gaps larger. So in cruise flight a greater degree of contraction is achieved by blowing cooler air (from the 4th stage of the compressor for example) over them.

Different engine types may use different sources of cooling air, but the general principle is the same.

13th Aug 2007, 22:34
Active clearance control provides a useful compromise by shrinking the casings by the correct amount to match engine running conditions. This is achieved by blowing cool air onto the outside of the casings. This cools the casings, causing them to contract, thereby reducing tip clearances.

Not quite KW.

The casing does not shrink (except during cooldown). The casing is still going to expand. The TCC system just controls the rate at which it expands until the turbine disc expansion catches up. That is where the timers come in.
BTW during high power maintenance engine running the TCC systems are deactivated. (stops us kack-handed throttle bashers trashing the motor dontchaknow). :ok:

14th Aug 2007, 08:41
Hello TURIN,

You are of course correct in that the casing isn't actually permitted to expand to its full un-cooled size, and then caused to shrink by blowing air over it. But the effect of the cooling is to keep the casing at a smaller diameter than it would have been without the cooling air.

I'm less sure about your emphasis on delaying expansion of the casing to permit the turbine disc to catch up. If it was simply a matter of delaying expansion then the cooling air would be used only for very short periods and not in any steady state condition such as cruise flight.

I'm not familiar with the specific details of the CFM56, but if ACC is used in steady state cruise (as some of the posts in this thread appear to suggest), then it cannot simply be a matter of allowing the turbine disc to catch up with the casing. The first few minutes of cruise should do that.