Steam Turbine Overspeed Trip Systems

By Boyd Davis

Abstract

Numerous failures and near failures of rotating equipment throughout history can be attributed to the malfunctions of overspeed protective devices. This can be due to lack of preventative maintenance or operators not having a clear understanding of the devices.

There is a need for everyone associated with rotating equipment to have a working knowledge of the overspeed protective device we know and have today.

In this presentation, the various overspeed trip devices and their operation will be discussed so that we all may have a better understanding of their purpose.

Introduction

In addition to operating speed governors, steam turbines are fitted with a shutdown system. Without proper control and adequate overspeed protection, catastrophic machinery failures can and do occur. The principal problems lie in the trip throttle valves; however, the entire system must be considered before any great improvement can be achieved.

How The Systems Work

The governor and overspeed systems vary from machine to machine and may be mechanical, hydraulic, electrical, pneumatic or combinations. Governor control systems consist of three basic elements. These elements are sensing, transmitting and correcting. Sensing elements may include fly ball weights, electric generator, and positive displacement pumps. Transmitting elements may be mechanical linkage, hydraulic or pneumatic pressure, electrical signals or, as is most common, a combination.

Sometimes an amplifying device such as a pilot, converter, or servomotor is necessary to boost the signal to a point where it can do useful work. The correcting element of the governor system is the valve or valves that control the flow of steam to the turbine. The valve for general purpose turbines is usually a single, double seated design, characterized by relatively high flows with low lift and low unbalance forces.

The desirable characteristics of a governor system are

1. Respond promptly to a small change in speed.

2. Adjust the throttle valve with a minimum of overshoot.

3. Have sufficient power to overcome friction losses and unbalance forces in the throttle valve.

4. Permit very little speed fluctuation under constant load and steam conditions.

There are several basic types of governors utilizing the above principles:

1. Mechanical shaft - The familiar fly weight type. A hand adjustment permits speed regulation at the machine.

2. Direct acting orifice - This consists of a shaft driven positive displacement type oil pump which delivers pressure to a spring diaphragm connected to the governor valve stem. Since the delivered oil pressure is directly proportional to shaft speed, control is accomplished. Hand or automatic speed regulation is possible.

3. Oil relay - Built to utilize lube oil pressure or a separate governor oil pressure, a double acting oil relay piston permits more precise control of the governor valve. It is integral to the turbine and usually designed by the turbine manufacturer.

4. Precision oil relay - A separate shaft-driven oil relay offers more precise control. Utilizing its own oil system, this type governor is not made by the turbine manufacturer. If more governor valve operating force is needed, a second double acting servo-motor may be utilized.

5. The Electronic governor usually provides more precise and reliable speed control. The speed measurement signal can be generated in two ways. One method is by utilizing a magnetic pickup in proximity to a toothed wheel/gear mounted on the turbine shaft. Another method is to utilize a shaft-mounted permanent magnet generator where the poles rotate and produce an electronic pulse measured by a microprocessor. In the first case, only two pairs of wires connect to the unit.

One supplies 48 VDC that is the required operating voltage; the other connects to a magnetic pickup on the turbine shaft. In the second case, no external power is required, as the unit is powered from the turbine shaft rotation. In the first case, output air (normally 3-15 lbs.) goes to the diaphragm of a standard control valve in the inlet steam line to the turbine. In the second case, hydraulic pressure drives a pre-piloted servo-motor that operates the governor valve.

Three internal adjustments are provided to set the operating speed and the gain and reset response of the unit. Electronically, the device is straightforward, consisting of a frequency to voltage converter providing the speed measurement that is compared to the internal speed set control. The difference is applied to the controller section where it will move the steam valve to hold the speed as desired.

Overspeed Trip Systems

In addition to a speed control system, steam and gas turbines are fitted with a shutdown system to prevent damage to the machine. In the event the speed governor fails to control the speed, the overspeed trip actuates to shut down the machine. When shaft speed exceeds a desired safe level, generally 10% overspeed, a latching device or oil dump mechanism is actuated to close a special emergency stop valve. This system is totally independent of the governor

There are two primary types of trip actuation systems, the mechanical type and electronic type. Figure No.1 shows a mechanical system that is completely separate from the speed governing systems. A trip pin or plunger is mounted in the turbine shaft with its center of gravity slightly off center. In the event the speed regulating governor fails to control the speed, the unbalanced plunger overcomes a spring force at a preset trip speed.

As it moves outward, it strikes the trip-lever, causing release of a spring dump valve that releases the trip circuit oil pressure. This unbalances a piston-spring combination and causes the trip and throttle valve to slam shut by the force of a spring and the steam pressure above the valve disk. A few high-speed machines use a weighted disk and a dished washer to accomplish the tripping action. The remainder of the action is identical.

In the electronic trip, speed is sensed similar to the system described in the governor section. When overspeed reaches the set point, an action is initiated to shut the emergency stop valve. This action is usually through an electric solenoid or mechanical valve that dumps the hydraulic oil on a trip throttle valve (large turbines) or releases a mechanical link to the emergency stop valve (small turbines).

In addition to overspeed, a solenoid valve can be made to shut down the turbine in response to low oil pressure, remote push buttons, or abnormal process conditions.

Basic Trip-Throttle Valve Designs

The design concept of the standard trip throttle valve is basically that of a globe valve with a stem nut that is mounted in a frame or bracket that is free to move. There are four design variations: two concerning the direction of the closing action, and two involving the method of holding the movable stem nut in its operating position.

Direction of Closing Action

The basic designs of the trip throttle valve with respect to direction of travel can be placed in two categories: (a) those where the valve plug is pushed onto the seat by the closing force, and (b) those where the valve plug is pulled onto a seat by the closing force. Because of the dual functions required of the valve - the tripping action and the throttling action - the stem must be in two pieces in both designs. The stem of the steam shutoff part of the valve does not rotate; it only slides to fulfill the tripping action needed.

The actuator assembly stem has rotary motion so that it can be positioned within the spring-loaded, hydraulically positioned stem nut to permit throttling. Therefore, there must be a change of direction and rotation within the split coupling. A hardened steel button, commonly called a thrust bearing, separates the ends of the two stems. Maintenance of alignment between the two stems is difficult.

Disk Is PUSHED onto Seat Design

In the larger valve sizes, the closing force on the valve stems and split coupling is not adequately designed to accommodate the impact load generated by this high closing force and any misalignment. This closing force must function in less than one-half second upon turbine overspeed, loss of oil pressure, etc. Frequently, damage occurs to stems or the split coupling with the plug pushed onto a seat design.

Disk is PULLED onto Seat Design

The design that "pulled" the plug onto the seat without a lower guide is the preferable design within the standard designs since the two stems and the split coupling operate in tension. This seems to limit the mechanical damage to the valve during the closing action.

Methods of Holding Stem Nut

Latch Type Stem Nut Holder Design - In this design, the bracket is spring loaded to push it in one direction, and has a knife edge latch mechanism to hold it and the stem nut in the proper position (See Figure No.2). When the valve is called upon to act as a trip throttle valve, the stem is latched in place and operates in a conventional manner, permitting raising and lowering of the plug. When the valve is called upon to act as a trip valve, the stem nut and bracket are released from their operating position on the knife edges by a small hydraulic piston and the spring pushes the stem nut downward so as to close the valve. "Hang up” of the hydraulic release piston is difficult to predict or prevent and is a major problem.

Piston-type Stem Nut Holder Design - Another type of trip throttle valve dispenses with the knife edge latch device and substitutes a larger oil cylinder. This is a globe type valve of inverted construction with the operating mechanism below the disk and a semi-balanced disk arrangement. In this valve, the force for closing the valve is provided by a main spring above the oil piston and the steam pressure above the disk (See Figure No.3).

After the valve has been tripped shut, turning the hand wheel clockwise resets it. The rotation of the screw spindle will raise the main piston and compress the spring. The hand wheel will be turned until the piston comes to rest against the cover and stops in an upward direction.

This valve has oil admitted through an oil inlet connection and orifice to the main oil cylinder with a relay valve. When the oil supply pressure is less than that required to reset the valve (generally about 50% of trip header pressure), the relay valve is unseated and the chamber below the main piston is opened to drain. When the oil pressure is increased to the reset pressure, the oil pressure on the relay piston overcomes the force of the relay spring, thus seating the relay valve and closing the passage from below the piston to the drain, permitting pressure to build up in the main cylinder.

To open the valve, the hand wheel is turned counterclockwise. The oil pressure will hold the main piston against the cover and the rotation of the screw spindle will lift the pilot valve off the seat. After the pilot valve is moved its full stroke, it contacts the disk flange and further movement unseats the disk. The valve should be backed off about two turns from the wide-open position.

When the oil supply pressure drops below the trip pressure (45 to 50% of normal pressure), the relay spring unseats the relay valve below the main piston to drain in the passage to the area above the piston. The spring and steam pressure closing forces will then trip the valve shut.

Some valves are equipped with an "exerciser” to check freedom of movement of parts. The exerciser is designed to limit the travel in the closing direction to a specified distance, normally less than 1/4 of the valve travel.

Operating Problems

The original designs of all trip throttle valves were based on the premise that the valve would be exercised through its full travel on a relatively frequent basis (2-3 month intervals). Most petrochemical plants do not operate in this mode. Serious doubts exist if any of these present trip throttle device designs can remain on the line for extended periods and be free to operate in an emergency. Most of the failure-to-trip conditions can be attributed to five basic problems.

1. Steam deposits on the valve stem (or stems).

2. Lubrication deposits (i.e., soaps, dirt, detergents, etc.) in the top works of the valve exposed to the elements.

3. Mechanical failures of the valve resulting from bent stems, either in the valve proper or the upper works, damaged split couplings, etc., all within about a 6" area near the center of the valve mechanism.

4. Galling of the piston in the hydraulic latch cylinder.

5. Jamming of the screw spindle in the larger cylinder-type valve design due to forcing by operations personnel.

Steam deposits present a problem either in the design that pulls the valve plug on the seat or the pushing force valve design. The latter quite often has an upper and lower guide bushing with tight fitting clearances. Both designs are then subject to movement retardation due to collection of steam deposits on the stem, which must enter into a tight fitting hardened bushing. In addition, the guide sleeve for both designs' steam valve tends to warp and offers a restraint to plug movement.

Because of the extensive sliding and pivoting actions needed to release the stem nut for the tripping action, lubrication of the valve upper works is necessary. This tends to either attract dirt and grit or retain moisture near tight fitting components. Exposure to high steam temperature also "cooks" out the lubricant, leaving "soap" or base material. All of these problems can cause binding of the linkages.

Experiences with various turbine installations indicate the following are good design practices.

1. The "pull on" plug design is better than the "push on" type.

2. A built-in "exerciser" is preferable. This feature is available only on the piston style valve. A significant percentage of valve hang-ups take place in the last one third of the plug travel so that any exerciser device might not move the plug far enough to clear the area of obstruction. However, even limited movement is better than ignoring the valve for extended periods. Manual exercising also has this limitation.

3. Most of these valves have less than 20 to 24 trips available before the valve must be disassembled for repairs. Often, all of these "good trips" are expended in setting the overspeed trip and other testing before the unit is placed in service. Trip repeat tolerance limits should not be too tight.

AP1-6 17 (Centrifugal Compressors for General Refinery Services) calls for trip speeds of 115% of RATED speed for compressor drives. This is 110% of maximum continuous speed. Tolerances are +2%. This gives trip ranges on the order of:

3,600 3,960+70 rpm (spread 140)

6,000 6,600+120 rpm (spread 240)

9,000 9,900+180 rpm (spread 360)

The trip itself and the latching mechanism are not designed for greater accuracy than this. Two or three repeats within these ranges are adequate. A great number of trips tend to damage the bolt and/or linkage and reduce its reliability.

Electronic Trip System

Electronic trip systems are highly desirable in higher speed units. The spring-loaded plunger becomes unstable at higher trip speeds and requires hydraulic relay valves to complete the tripping action.

The mechanical system requires extra shaft overhangs (detrimental to vibration on high-speed machines) and uses hydraulic fluid in long piping runs. An electronic system measures pulses from a toothed wheel on the turbine shaft and puts those pulses through a frequency to voltage converter. That output goes to a comparator that is switched at the set point and is coupled to a power amplifier, which trips an electric relay. The relay dumps a solenoid valve, which dumps trip header pressure. From here, the system is the same as the mechanical system.

The advantages of the electronic system are that (a) it requires only short shaft overhangs, (b) it is fast, accurate, and easily adjustable, (c) it allows great flexibility of system arrangement, (d) it can be easily tested without running the machine, and (e) it gives a high degree of repeatability.

The disadvantages of the electronic system are that (a) it requires an additional reliable power source and (b) it requires explosion-proof classification in many applications.

Test Running of the Turbine

The ease of operation of the electronic governor systems makes it very convenient for the operator to use and to bring the turbine up to speed. Under normal operating conditions, this poses no problems, but when slow rolling, test running or overspeed testing the turbine while uncoupled from the load, control should be by the TRIP THROTTLE VALVE ONLY. The slightest movement of the governor valve in an uncoupled situation can result in tremendous changes of speed. A stuck governor or linkage can be very dangerous. If it is necessary to prove out the governor system, limit the maximum speed with the hand wheel of the trip throttle valve and control the speed downward with the governor system.

Conclusion

Due to the speeds and demands that rotating machinery is subjected to today, there is a demand for a new design concept. The present mechanical hydraulic system should be evaluated for its reliability and replaced with a hydraulic system that minimizes all of the mechanical functions as much as possible. The entire trip system needs to be reviewed and made more reliable.

Serious consideration should be given to eliminating the dual trip and throttle valve system on larger turbines operating (500 hp and up) and installing a separate overspeed trip valve that is direct acting, unbalanced and has one stem. The balanced type throttle valve has the tendency to slow down the closing time of the valve required to prevent the rotor from accelerating beyond the set trip speed that could result in mechanical damage.

Every trip valve should be equipped with an exercise so that the valve stem can be periodically stroked to approximately one third of the closed position, and any deposit built up on the valve stem due to solids in the steam can be removed. This travel should be controlled by some type of limiting travel relay to prevent over-travel.