Rupture Pin Valve Application in mill plants

Rupture pin valves are environmentally sound pressure relief devices that can help mills reduce downtime in the bleach plant, pulp mill, and powerhouse areas.
Domestic competition and the growing amount of paper imports continue to challenge North American mills to improve product quality while at the same time reducing operating expenses. One of the most obvious ways to increase profits is by reducing mill downtime, and some mills have turned to an easily maintained pressure relief device that is helping accomplish this goal for various applications in the pulp mill, bleach plant, and powerhouse areas.
The rupture pin valve is an ASME-approved device that can help mills limit downtime, while offering environmentally friendly pressure relief.
For pulp processing, the rupture pin valve is used on continuous digesters, batch digesters, and white liquor pressure filters. Each of these vessels is kept under pressure and must have a dependable pressure relief device, and the rupture pin valve offers the reliability and dependability necessary to help maximize runtime. The most commonly used valve for this application is the Rupture Pin Model C valve. This valve has a 90? angle and is easily installed on the vessel or relief piping. The valve can be installed on the side of the vessel with the relief lines running along the side of the vessel.
In the bleach plant, operators have found the Model CM rupture pin valve fits well on all the pressurized reactor vessels. This valve has an extended piston that fits down into the vessel, preventing pulp from clogging the relieving area and a flush port so the seat area can be flushed for clean resetting of the valve when there has been an over pressure situation (Figure 1).
The up flow lines on all bleach plant vessels should have the rupture pin valve protecting the lines from problems associated with blockage. When a blockage occurs on the up flow lines, the rupture pin valve is faster to relieve building pressure than a rupture disk, therefore offering a lower chance of line damage.
The third area of the mill where the rupture pin valve has eliminated downtime is probably the most influential: the non condensible gas (NCG) and sulfur off gas (SOG ) lines to the powerhouse. The premature failure or unscheduled relief of the rupture disk on the NCG lines can create safety, environmental, and manpower issues. These issues are minimized with the rupture pin valve, because the integrity of the system is not compromised. There is also no need to break out chemical suits, toolboxes, and gather the maintenance crew. You can send one person with a replacement pin, if the spare pins are not stored at the valve, and the person will return in a matter of minutes ready to resume production. Government required regulatory paperwork is also reduced, because the line is never broken apart, releasing excess gases.

How the Valves Work
The Rupture Pin Technology pressure relief valve is a simple yet high tech device. The valve only has one moving part, the piston. With its elastomer seal, the piston seals the inlet port.
System pressure is measured on the face of the piston. The system pressure is then transferred to the piston, which is held in place by the pin. When the system pressure increases to the valve set pressure, the pin will buckle according to Euler\'s Law of Compressed Columns. The valve will have a bubble tight seal until the pin buckles, at which time the system will have full bore relief.
The valve is easily reset by removing the pressure at the valve, reseating the piston by depressing the piston stem, removing the buckled pin, and installing a new pin. The pin is external and is not affected by fluid corrosion or temperature. The pin buckles by Euler\'s Law, not when the material fails in compression. Therefore, the pin cannot fatigue because the pin buckles before the elastic limit of the pin material is reached.
The pin provides a good indicator of valve position. If it is straight, you have a bubble tight seal, but if it is buckled, you are in the relieving position. The optional proximity switch will send a signal alerting operators of valve action. The valve is accurately repeatable and will not experience premature failures normally associated with alternative pressure relief methods and rupture disks. Since the pin is in compression, fatigue and pulsation have no effect on the valve or the set pressure of the valve.
The key to the system is the pin. The diameter is precisely centerless ground and measured to five decimal places, while the pin length is measured to four decimal places. For a specific set pressure, Euler\'s Law dictates the diameter and length of the pin.
Pin material is ordered in bulk, and it is important that it comes from the same heat treatment batch. When the material arrives, it is essential that the material have the same uniform modulus of elasticity. Lot qualification tests are run on a Tinius-Olsen test machine and the results are compared with the previous batch for buckling consistency. These tests are performed in a compression test machine capable of a 60,000-lb axial force.
The compression test method used is free of any seal friction to obtain accurate and repeatable results. Software graphs the axial force versus time to show the ultimate load at which the pin buckles. The variations in these tests are less than 1%. Without this information, accurate pins cannot be reproduced when replacement pins are needed.
Tests on actual valves are to be accurate within +/- 5% of set point to meet ASME requirements. Seal friction is the cause for set point variations in actual valves. Test certificates for the rupture pin valves have shown variation that is usually below +/- 3%.
Maintenance on the rupture pin valve is minimal. The ASME classification for the valve is a pressure safety element, meaning it does not require annual recertification. The objective of maintenance is to ensure that the piston is free to move. If the piston does not move, the o-rings need replacing. The maintenance on the pin is visual inspection and rolling on a flat surface to ensure straightness.

Results from Mill Installations
Of the North American mills using the rupture pin valve, a west coast mill reports that it has already begun saving money since the October 2004 installation in its DlOO vessel in the bleach plant.
According to the west coast mill\'s bleach plant maintenance planner / supervisor, the mill had been using a rupture disk from another supplier. While he reports that it was a good product, he also says the time to change the disk is 10 times as much as the time to change the rupture pin. In addition to easy maintenance, the supervisor was impressed with the \"lack of preventive maintenance required on the pressure relief device.\"
A southeastern mill has installed the rupture pin valve on the NCG lines leading to its powerhouse. The mill had previously used rupture disks on the lines. Whenever the rupture disks would fail prematurely, four maintenance specialists would have to put on chemical suits for each change out. The time necessary to change the disk was 4-6 hours for each premature failure.
The rupture pin valve was installed at the southeastern mill in June 2004. According to the plant maintenance supervisor, the valves have helped eliminate premature failures and any unscheduled shutdowns related to the NCG lines. He was also relieved about not having to report any environmental issues to regulatory agencies due to the \"nuisance trips\" caused by the rupture disks.

LEE LEVISAY is international sales manager for Rupture Pin Technology in Oklahoma City, Okla. The author would like to acknowledge Al Dunham of New England Controls in Bangor, Maine, for his help with this article.
Copyright Paperloop, Inc. Sep 2005
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