Curing Time for Gas Turbine Foundation 1

[norzul]

Hi,

What is the typical curing time for the foundation. Fyi, we are constructing a foundation for the 25MW gas turbine..

Thanks

norzul

 

[SlideRuleEra]

Wet cure for 7 days is usually sufficient. Be careful however, setting equipment on the foundation is dependent on concrete strength, not the cure time.

http://www.SlideRuleEra.net

 

[boo1]

We use three days for light foot traffic, seven for moderate traffic, 28 full loading. The normal moist cured concrete seven day strength, using various types of cement it ranges from about 50% to 78%.

Type I at 70 degrees F
f'c(t) = f'c(28) * { t / (4 + 0.85t) }
where f'c(t) represents the compressive strength at age t and f'c(28) represents the specified 28 day strength.

 

[norzul]

Hi,

I was informed that the bigger the foundation i.e. more mass, the stiffer it will be and the higher its natural frequency will be. Is this correct?

We just procured GTG (gas turbine + generator). The machine will be located on the same block of foundation.
1)Dimesion : 21m (length) X 2.5m (width) X 4.5m (depth).
2)Total mass of the GTG is +/- 220 tons. GT : 120 ton; Generator : 100 ton
3) Total mass of foundation : 880 tons
4)Material of foundation is reinforced concrete
5) GT speed : 5100 rpm (85 Hz)
6) Generator speed : 1500 rpm (25 Hz)

Was also informed that:

There are two strategies to design turbine foundations:
1)"low-tune" or "high-tune"
2)High-tuning would lead to impractical thickness of the foundation block.
3)For this reason, low-tuning has been adopted resulting in fundamental structural natural frequency below machine operating frequency.
4) There are two standards and requirements provided by the vendor which are used to assess the acceptability of the calculation results. The standards are:
- German standard DIN 4024, Machine Foundations; Elastic Supporting Structures for Machines with Rotating masses /2/
- International Standard ISO 10816-2, Mechanical vibration - Evaluation of machine vibration by measurements on non-rotating parts - Part 2 /7/

The natural frequency of the foundation is calculated to be at 19 Hz. The acceptable range as specified by the standard is below 20Hz and above 28 Hz. Is this commonly done? coz the generator supplier insist the natural frequency to be above 28 Hz i.e. resulting in more mass of foundation...

 

[cvg]

If your foundation design is not acceptable to the supplier, will the warranty be void? If so, you might want to listen to them.

 

[SlideRuleEra]

High-tune foundations are thick, but they are not impractical for 60 Hz machines. The fact that your's operates at 25 Hz probably makes the difference. As cvg says, the turbine-generator manufacturer sets the rules for vibration requirements - suggest you take the manufacturer's advice.

http://www.SlideRuleEra.net

 

[norzul]

Actually I'm represent the client. We deal with the EPCC (Engineering,Procurement,Construction,& Commissioning) Contractor. It is a lump sum contract. The contract is between client and EPCC Contractor. The gas turbine package is supplied by GE/NP which is under EPCC Contractor. The package includes the generator which is manufactured by Brush.

The foundation is designed by the EPCC contractor and they are 110% confident with the design. Technically, GE/NP has got no problem. Only Brush, the generator supplier has some concerns regarding the foundation design.

It is still under discussion, but most likely we may proceed with the EPCC Contractor's design i.e. low tune design....