2.
认hirling mechanisms such as:
a. Oil whirl
b. coulomb whirl
c. Aerodynamic cross coupling whirl
d. Hydrodynamic whirl
e. Hysteretic whirl
3. Blade and vane passing frequencies
4. Gear mesh frequencies
5. Misalignment
6. Flow separation in boundary layer exciting blades
7. Balljrace frequencies in antifriction bearings usually used in aero-derivative gas turbines
Torsional criticals should be at least 10% away from the first or second harmonics of the rotating frequency. Torsional excitations can be excited by some of the following:
1. Start up conditions such as speed detents
2. Gear problems such as unbalance and pitch line runout
3. Fuel pulsation especially in low NOx combustors
The maximum unbalance is not to exceed 2.0 mils (0.051 mm) on rotorswith speeds below 4000rpm， 1.5 mils (0.04 mm) for speeds between 4000-8000rpm， 1.0 mil (0.0254 mm) for speeds between8000-12，000 rpm， and
0.5 mils (0.0127 mm) for speeds above12，000 rpm. These requirements are to be met in any plane and also include shaft runout. The following relationship is specified by the API standard:
.
Lv二 12000 (4-1)
N
where:
Lv二Vibration Limit mils (thousandth of an inch)， or mm (mils x 25.4) N二 Operating speed (RPM)
The maximum unbalance per plane (journal) shall be given by the follow-ing relationships:
Umax二 4W/N (4-2)
where:
Umax二 Residual unbalance ounce-inches (gram-millimeters)
W二 Journal static weight Lbs (kg)
A computation of the force on the bearings should be calculated to determine whether or not the maximum unbalance is an excessive force.
The concept of an Amplification Factor (AF) is introduced in the new API 616 standard. Amplification factor is defined as the ratio of the critical speed to the speed change at the root mean square of the critical amplitudes.
AF二(N2 N-c1 N1)(4-3)
Figure 4-6 is an amplitude-speed curve showing the location of the run-ning speed to the criticalspeed， and the amplitude increase near the criticalspeed.认hen the rotor amplification factor， as measured at the vibrationprobe， is greater than or equal to2.5， that frequency is called critical and the corresponding shaft rotational frequency is called a critical speed. For thepurposes of this standard， a critically damped system is one in which the amplification factor is less than 2.5.
Balancing requirement in the specifications require that the rotor withblades assembled must be dynamically balanced without the coupling， but with the half key， ifany， in place. The specifications do not discuss whether this balancing is to be done at high-speeds or low-speeds. The balancing conducted in most shops is at low-speed. A high-speed balancing should be used onproblem shafts， and any units， which operate above the second critical. Field balancing requirements should be specified.
The lubrication system for the turbine is designed to provide both lubrica-tion and cooling. It is not unusual that in the case of many gas turbines the maximum temperatures reached in the bearing section is about 10-15 min-utes after the unit has been shutdown. This means that the lubrication system should continue to operate for a minimum of 20 minutes after the turbine has been shutdown. This system closely follows the outline in APIStandard614， which is discussed in detail in chapter 15. Separate lubrica-tion systems for various sections of the turbine and driven equipment may be supplied. Many vendors and some manufacturers provide two separate lubrication systems: One for hot bearings in the gas turbines and another for the cool bearings of the driven compressor. These and other lubrication systems should be detailed in the specifications. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：燃气涡轮工程手册 Gas Turbine Engineering Handbook 1(60)
