VALVE REDESIGN BLOCKS BROKEN-BOLT DAMAGE

Richard A. Mueller
OXY USA Inc.
Tulsa

Redesign of a valve chair in a suction valve on a KVS 2,000-hp compressor at a Texas gas plant has prevented potential damage from broken bolt fragments entering the valve area of the cylinder.

Additionally, the jackbolt metallurgy was changed to a nitrogen-strengthened austenitic stainless steel (Nitronic 50).

FAILURE, ANALYSIS

OXY USA Inc.'s Lefors, Tex., gas-processing plant has two KVS 2,000-hp compressors which operate with three stages of compression.

Nominal suction and discharge pressures for each cylinder are as follows: first, 13-in. vacuum and 28 psig; second, 28 psig and 115 psig; and third, 115 psig and 600 psig.

The residue gas is reinjected to maintain formation pressure. The gas contains about 0.21 % CO2 and 0.50% 02 with only traces of H2S as impurities.

In the summer of 1988, one of the KVS 2,000-hp compressors suffered failure of a third-stage suction valve jackbolt. A piece of the bolt fell through the valve port into the cylinder.

This occurrence, proving that steel is incompressible. caused considerable damage to the cylinder. Failure cause of the bolt was traced to corrosion-initiated fatigue.

Fig. 1 shows a typical cross-sectional view of a valve assembly held by the chair, jackbolt, and valve cap. The fracture plane of the jackbolt is at, and flush with, the inner surface of the valve cap.

The fracture surface of the bolt is shown in Fig. 2. The surface's "beach" marks are indicative of a bending fatigue crack-propagation mechanism.

Metallographic cross-sections were taken, and a typical photomicrograph of the threads at the crack origin is shown in Fig. 3. Cracks at the thread roots are evident.

Other photographs revealed some cracks were filled with corrosion products and appeared intergranular.

It is hypothesized that some form of stress corrosion cracking (SCC) initiated the cracking which then converted to fatigue propagation. The SCC-corrosion mechanism most common for carbon steels is "carbonate-bicarbonate," which is intergranular in nature.

CO2, O2, and H2O are all present in the gas which gives the correct elements. This mechanism is usually associated with cathodically protected pipeline SCC in soil. Hydrogen sulfide is only present in traces; therefore, wet sulfide cracking mechanisms are not applicable.

A 410 stainless-steel jackbolt was tried as an interim solution, but it too developed cracks at the threads. Research to identify the exact mechanism was not conducted.

STRESS CONSIDERATION; ALLOYS

Cracking occurs as a result of tensile stresses.

Although the jackbolt is nominally loaded in compression, tensile stresses can develop in two ways:

• If the locknut on the outside of the valve cap is over torqued, the tensile loading can develop in the threaded portion of the bolt between the inner and outer surfaces of the cap.

• The valve chair bearing surface can crush, allowing the bolt to become loose. Sideways vibrations can create bending loads with tensile components on outer surfaces.

If a bolt were loaded as for a cantilever beam, stresses are highest at the failure location.

For stress computations, a 500-psig pressure difference between suction and discharge pressure and a 500 ft lb torque on the bolt were used to give a total compressive force of 37,892 lb.

The bending stress due to this load was computed to be only 3,805 psi, which is low. This further indicates crack initiation began other than by fatigue. Because both carbon steel and 410 stainless steel bolts failed, an upgrade to an austenitic grade was made.

The Nitronic 50 grade (UNS 520910) was chosen because it has good corrosion resistance and a tensile strength of more than 100,000 psi, compared with only 75,000 psi for 304 stainless steel.

VALVE CHAIR

A new valve chair design was conceived by the plant manager working with a compressor repair contractor.

The new design permits the jackbolt to be 1-in. shorter, and the chair extends past the outer lip of the valve intake manifold of the cylinder. This design lowers the bending stresses by shortening the cantilever arm and prevents a bolt fragment from entering the valve area if the bolt were to break.

A drawing of the new chair is shown in Fig. 4.

A prototype of the chair was made and fitted to a spare cylinder at the plant. Final dimensional corrections were made and the new chairs and jackbolts installed.

After more than a year's service since the new chairs and jackbolts were installed, no jackbolt failures have reoccurred.

ACKNOWLEDGMENT

The help of Terry Sims, Lefors plant manager, and Panhandle Industries, Pampa, Tex., is appreciated.

Copyright 1991 Oil & Gas Journal. All Rights Reserved.