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  Webinar Archives > Shock Control Devices

Shock Control Devices Presentation (May 2009)

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Text Version of the Shock Control Devices Presentation

For those unfamiliar with us, we would like to share some background information on our company. Our parent company, Piping Technology & Products, Inc., also known as PT&P, has been in business since 1975. PT&P and its wholly owned subsidiaries, US Bellows, Sweco Fab, Pipe Shields, and Anchor Darling, offer a wide range of engineered products and services for various industries and applications. Our product line is extensive, from spring supports, expansion joints, pre-insulated pipe supports, and miscellaneous fabrication to various engineering and technical services, PT&P has decades of experience providing products and services for all your engineering and construction needs.

We have achieved various certifications here at Piping Technology & Products. We have the ASME U-Stamp and R-Stamp for the SWECO division of our company. We are ASME NS certified in our Fronek Anchor/Darling, Enterprises division. We are also part of the Expansion Joint Manufacturers’ Association through our U.S. Bellows, Inc. division. Most recently we received the ISO 9001:2000 certification for the manufacture of hot and cold supports. We are a Minority Business Enterprise granted by the City of Houston, and additionally, we are a member of the Houston Minority Business Council.

This webinar will focus primarily on shock control devices. We manufacture three main types. The first shock control device is the snubber. The most common Custom designed hydraulic snubber for maximum load of 140,700 lb.example of a snubber in working principle is the automotive safety belt. They are not activated under normal thermal contraction and expansion. They allow movement in tension and compression under normal conditions, but activate when impulse events occur. There are hydraulic and two types of mechanical snubbers. They transmit the forces to a fixed base while acting as a rigid restraint. Another type of shock control device is the sway brace. Sway braces use springs to provide restraint force while allowing movement in tension and compressions. The restraint load is proportional to travel. The last shock control device is the sway strut. Sway struts are rigid supports, but do not allow movement in tension and compression. It does allow for rotational movement or 1 degree of freedom.

Snubbers are used as restraining devices to safeguard the piping system against impulsive loading while introducing negligible resistance during normal thermal movement. Undesirable abrupt movements of the components can be caused by either internal or external excitation. Internal excitation consists of pressure shocks from valve operation, water hammer, boiler events, turbine trips, pipe breakage, and safety/relief valve discharge. External excitation is caused by wind load, seismic or cyclic events, impact by moving vehicles, and explosions. Under normal conditions, slow thermal movement of the piping system does not activate the snubber. Typically, an impulse type event will occur for a short time, but with a large magnitude. As the piping system responds, the impulse is transmitted to the snubber through a support-member. When the impulsive force exceeds the predetermined threshold value, the snubber activates, becoming a rigid support and arrests axial movement transmitting the shock through another support member to the fixed base. (NOTE: piping system will not move)

Hydraulic snubber being utilized in the field.In a sudden shock event, an instantaneous, secure and almost rigid connection is produced. The resulting dynamic loads are at once conducted into the structural connection. Normal operational movements of the piping and other components remain unhindered after impulse loading. In the case of seismic events, typical frequency response is 1 to 33 Hz. The two main types of snubbers are hydraulic and mechanical. The design criteria vary. However, there are a few common design requirements. First, snubbers must be able to move back and forth, so that it can lock the motion and resist inertial forces of oscillatory nature in both directions along the longitudinal axis of the snubber. Next, during normal conditions, snubbers shall allow essentially free movement of component imposing minimum resistance when the component is subjected to non-dynamic loads such as those imposed by thermal expansion and contraction. Third, the force required for axial movement shall not exceed 1 to 2 % of the rated load. Fourth, a snubber should lock or produce large inertia force when the linear velocity of the piping system supported by snubbers exceeds design criteria, or acceleration exceeds the design value. Next, the peak to peak displacement across the unit, including end attachments, shall not exceed 1-30-40 Hz, when input frequency is in 133 Hz range at nominal load. Sixth, the snubber shall be designed for normal operation at 120F. Seventh, the snubber shall be designed for normal operation at relative humidity of 25%. Finally, the materials shall not promote corrosion under the operating conditions.

Suppose that a piping system moves correctly through a hydraulic snubber under normal conditions. As the first piston moves forward, it pushes the fluid through the channels. As long as the piston velocity, V < V critical, the valve remains open. The fluid pushed by the piston flows uninterrupted through the channels. The fluid enters into the back of the main cylinder through a port and also to the compensating cylinder through another port. The same uninterrupted flow occurs through the valves and channel if the first piston rod moves in the opposite direction (in this case, the flow through the channel is reserved). Such movement occurs due to thermal expansion and contraction of the piping system. During this free movement, the breakaway drag is typically 1-2% of the rated load.

The locked position of a hydraulic snubber is similar to the normal condition. When subjected to movements that exceed the design velocity limits (which is typically 8" min.), the valve is activated and is momentarily closed. This is known as the lock up of the snubber. This forms a rigid connection between the piping system and the support structure. When the load has dissipated, the snubber unlocks and again allows gradual movement of the pipe and component. During the lock up state, a small amount of fluid is allowed to bleed either through a narrow channel or through a separate channel in the valve itself.

Exploded view of a hydraulic snubber.The hydraulic snubber is composed of many different parts, but the high pressure main cylinder is offered in seven cylinder bore sizes from 1" to 8". The selection of size depends on the anticipated force and thermal movement of the protected piping/equipment. Our hydraulic snubbers include a flow control section that contains dual stage velocity sensitive poppet valves, which are all made to include reservoirs in 6", 10", 15", or 20" strokes, except the 1" bore size (The 1 1/2 inch bore size is offered only in 6 and 10 inch strokes). A spherical bearing allows for angular rotation of 6 to accommodate misalignment.

Piping Technology and Products, Inc. offers several optional design features. This first is an integral relief valve (inside), which is a non-adjustable valve that is factory preset at 133% or 200% of rated load. The next feature is the protective boot. The boot is installed over a piston rod for protection against corrosive or outdoor environments. The systems movement is controlled by a flow control device, which can be designed as adjustable if needed.

Piping Technology & Products, Inc. manufactured a custom hydraulic snubber with an adjustable length for a power plant in Turkey. Most snubbers have a fixed cc dimension, but these are custom designed with a lock nut that locks in place after adjusting the length by removing the rod end screws in and out of the cylinder. It is capable of a maximum load of 140,700 lb. and the stroke is 6". It was designed for 375F with a pressure of 1750 PSIG. Another custom job was a carbon steel hydraulic snubber capable of loads of 225,000 lb. It has a three-coat paint system with a 14" diameter and weighs 4000 lb.

Snubbers are used in piping systems or equipment when unrestrained thermal movement must be allowed under normal operations, but must be restrained during impulsive or cyclic disturbance. They essentially restrain shock loading both in tension and compression. Special settings are available to absorb the continuous thrust resulting from safety valve blow-off or pipe rupture. For the most effective operation of the unit, please specify the mounting position, vertical or horizontal. Preferred usage with standard settings is to prevent destructive results due to earthquakes, flow transients or wind load. The unit is not effective against low amplitude, high frequency movement.

PT&Ps MSA series mechanical snubber operates on the principle of limiting the acceleration of any pipe movement to a Variety of mechanical snubbers.threshold level of 0.02g. When a disturbance occurs that exceeds the threshold, the ball screw and drum produces angular momentum to the inertia mass. The inertial resistance of the mass engages the resilient capstan spring to tighten around a hardened mandrel, which is part of the structural tube. This in turn causes restraining force against rotation of the ball screw. During thermal compensation, the associated acceleration is far below the threshold limit of g (in other words, any slow movement will not activate the capstan spring). Previous Example: A seat belt or complete arresting of motion.

In addition to MSA mechanical snubbers, PT&P also designs and manufacturers dynA/Damp mechanical snubbers. Depending on the load and configuration, the snubber becomes more complex with more numbers of oscillating verges and gears. Instead of locking, the dynA/Damp mechanical snubber produces increasing resisting inertia force with increasing input velocity produced by external force. It utilizes inertia force of oscillating verge(s) with connecting gearing, which limits the velocity of the geared rack and thereby controls the rate of linear displacement of piping or other equipment systems. During normal thermal movements, the rack is free to displace with relatively small resistance. Upon introduction of an external applied force such as a seismic event oscillation of the verge limits movement of the geared rack to a velocity proportional to the applied load. Since the snubber is always engaged, instant snubbing action assures positive displacement control of the connected piping or equipment system. For example, we manufactured several mechanical snubbers for a refinery. These snubbers ranged in size from 18" to 80" in length in the retracted position. Their maximum load capacity was 700 to 10,000 lb. These snubbers were tested at both high and low rates of loading on a computer controlled test machine.

Mechanical snubbers have several advantages over hydraulic snubbers. They are maintenance free and can be placed in any orientation; including, vertical, horizontal or skewed. There are standard strokes readily available and can be used in a series to increase the available travel range. The compact design of the mechanical snubber allows for small overall length. Sizes range from to 100, load rating is from 350 to 120,000 lb. and the standard maximum stroke ranges from 4 to 6 inches with an effective stroke of 12 inches. Another example is a recent order we had for 61 mechanical snubber assemblies for a power plant. These assemblies are used to control a maximum load of 1500 lb. within a piping system. Each snubber was supplied with a custom designed yoke clamp made from chrome-moly, and a stainless steel u-bolt, and a protection saddle. Their travel capacity range was 1-5". Mechanical snubbers have several key features. One of the features includes an adjustable length for field fit-up. Another feature is that mechanical snubbers can be used in conjunction with a yoke clamp. They also allow movement in axial and translational directions. Finally, they can restrain all movement when a disturbance even occurs.

Under normal operating conditions, mechanical snubbers are generally maintenance free. Under severe service, seals may become damaged, causing a loss of fluid. In this case, the snubber must be completely disassembled, new seals installed, and replenished with new fluid. Severe service usually involves excessive vibration or high temperatures. If the snubber is damaged, contact us immediately for a replacement. If the snubber needs to be refurbished, remove the snubber from operation and send it back to us for maintenance and testing.

Snubber testing machine at PT&P.PT&P has two test machines specifically designed for testing snubbers for piping applications. One machine is used for routine testing of hydraulic snubbers, and a more sophisticated machine performs a wide range of tests on both hydraulic and mechanical snubbers. The STADAS is a trailer mounted snubber test machine manufactured by Paul-Munroe Inc. of California. This computer controlled, hydraulic test machine is used to test the performance of mechanical and hydraulic snubbers ranging in size from 150 lb. to 135,000 lb. The STADAS may also be used to verify snubber conditions at the point of manufacture, or during an outage (refueling) at the plant site. The STADAS machine is capable of controlling a test and follows any load sequence needed. It can then plot the test results immediately, and save the results to disk for future reference. The main frame of the STADAS machine is comprised of two drive cylinders, which supports the snubber being tested. The smaller drive cylinder is used for snubbers rated to 6,000 lb., and the larger drive cylinder is used for snubbers rated up to 150,000 lb. Each drive cylinder is equipped with an electro-hydraulic servo-valve, load cell and connections to the hydraulic system.

The hydraulic snubber test machine is mainly used to perform testing for hydraulic snubbers. It has a similar test bed like the STADAS machine. However, it is not computer controlled. It is adequate to perform all the tests normally required for hydraulic snubbers, such as quality control and periodic tests that are sometimes required as part of plant maintenance programs. Mechanical snubbers can also be tested on this machine, but due to the complication of the process, mechanical snubber testing is usually performed on the STADAS machine.

Sway Braces are recommended for absorbing shock loadings, guiding or restraining the movement of the pipe resulting from thermal expansion, and bracing a pipe line against sway. The working principle is similar to variable springs; as movement increases, the restraint force increases. It can be utilized in tension or compression or a combination of the two. Standard sway brace sizes have 3" of total travel in tension or compression and load ratings from 50 to 7200 lb. For example, ten sway braces were designed and manufactured for a petroleum company. Their total travel distance is 14" while our standard PT&P sway braces have springs that allow 3" of total travel distance. These sway braces were custom designed to provide a total travel distance of 5" and 14". Their center-to-center dimensions ranged from 92 " to 115" with internal supports to prevent buckling. The desired spring rates for the sway braces was 450 lb./in. and 1,350 lb./in.

Recently, Piping Technology & Products, Inc., designed and fabricated 126" long sway braces for a power plant. These PT&P sway braces are fabricated from carbon steel and measure 126" from pin to pin. They use an integral piston assembly with guides to eliminate any buckling and/or misalignment during extension and compression. The sway braces are designed for 10" of travel and a load of 1,500 lb. A combination of standard spring coils is used to achieve the required spring rate of 150 lb./in. Prior to shipment, a test was performed using a hydraulic ram. This test simulated actual field conditions and allowed the sway brace to move through the entire travel range in both compression and extension modes. Another example is a project where we manufactured custom sway braces for a local refinery for stabilizing tall stacks. These sway braces have a deflection capability of 9" in all directions and ranged from 71" to 144" in length with load capability of 8000 lb. They include machined ball joints at each end that allow for freedom of movement in any unpredictable direction. Some sway braces use an integral piston assembly with a guide to eliminate any buckling and/or misalignment during extension and compression. We use a hydraulic ram to simulate actual field conditions and allow the sway brace to move through the entire travel range in both the compression and extension modes in order to test our products.

Sway strut assemblies are used to restrain movement of piping in one direction while providing for movement due to thermal expansion or contraction in another direction. They are effective for either tensile or compressive force. The self-aligning bushings permit 5 misalignment or angular motion.

Sway strut assemblies with pipe clamps.For example, PT&P fabricated some sway strut assemblies with custom designed 3-Bolt pipe clamps. These sway strut assemblies were designed in conjunction with custom three-bolt clamps to prevent movement on a pipeline. The sway struts range in length from 25" to 70" from pin to pin. The operating load ranges between 8,000 lb. to 15,000 lb. and the clamps were designed for pipe sizes ranging from 8" to 24". The sway strut assemblies are fabricated entirely from carbon steel with a hot-dipped galvanize finish. The completed assemblies also include a custom 2-coat paint system that was applied over the galvanize finish before being shipped to an LNG plant overseas. Another example, are two chrome-moly sway strut hangers that we fabricated for a power generating station. These sway struts are made of ASTM A387 GR22 and weigh 1,485 lb. each. They are 6" in diameter and 8'2" long and are used for main steam valve supports with specially designed spacers, pins, and retainers capable of restraining movement up to 90,000 lb. at high temperatures exceeding 750F. A third example is a sway strut we fabricated from carbon steel and is 6 in diameter with a 15,000 lb. load. The sway strut includes a hot-dipped galvanize finish and will support a 30 process line in a refinery. This particular product was necessary because the current line only had a few i-beams for support and was not able to handle the large forces acting on the line. A heavy schedule pipe and heavier duty strut ends were necessary due to the heavy loads acting upon the pipe.

Piping Technology & Products, Inc. offers an emergency service for rush orders. Simply visit our website and click on emergency at the top-right corner of our home page. We guarantee a response within 30 minutes. That concludes our presentation on shock control devices. For any questions you may have, you can email us at info@pipingtech.com.

 

 

 

 

 

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Mailing: P.O. Box 34506, Houston, TX 77234-4506 Location: 3701 Holmes Road, Houston, TX 77051
Our Subsidiaries: U.S. Bellows: Metallic & Fabric Expansion Joints, Bellows
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: Snubbers, Sway Struts & Sway Braces
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