Flanges and Clamps — Should They Leak?

June 30, 2009 – 8:09 PM

Article written by Cliff Knight of KnightHawk Engineering.

Offshore-flange-joint-leak

You’re offshore on one of the largest rigs in the world. It was put together with all the latest equipment with all the “bells and whistles.” The rig was built, erected and went into production in record time. As with any new unit, it takes a while to get all the “bugs” out, tune things and get lined out. While observing the rig, you see something you don’t like — process fluid dropping out of a flanged joint. The flange connection is a new quick connect type joint (QCJ). The maintenance crew chief is called and he sends someone out there to tighten the bolts on the QCJ, and the small leak stops. You’re relieved and in subsequent observations, you see no leak. A few weeks later, you walk by and you see “drip, drip, drip …” You call maintenance again, they tighten the clamp, and the leak stops. Once again, you don’t see it at first, but it pops up again in several weeks. You ask, “What’s going on?”

Your company has approved the QCJ for this service; it has been tested, and it is used widely all over the company. The QCJ comes from a well-respected manufacturer, and your company has had a long relationship with them. You ask yourself, “What is the problem” and call for help.

Read More »

By Alex | Posted in Inspection & Testing, Technical Information | Tagged , , , | Comments (1)

Protecting Progressing Cavity Slurry Pumps

June 29, 2009 – 3:44 PM

Article written by David Gardellin of Onyx Valve Co

A progressing, or progressive cavity pump consists of a single helix metal rotor that turns inside a double helix rubber stator. This forms a series of pockets that traverse the length of the pump, gently pushing fluid from the suction to the discharge in a smooth, pulseless stream.

Progressing-cavity-pump

Progressing cavity pumps are unmatched in their ability to transport viscous, abrasive, and shear sensitive fluids. They can pump exotic solutions such as shrimp and brine without crushing or tearing the product. They routinely handle sewerage sludge, ceramic slurry, wax, chemicals, confectionery ingredients, grinding compound, sand, cement, grout, putty, pulp, ground meat, pigments, glue, paste, grease, paint, lime slurry, jelly and soap.

Their precise internal geometry and minimal back flow make them accurate metering pumps, eliminating the need for a flow meter. You can deduce volumetric flow rate by adding a tachometer pickup to any rotating part of the pump and motor assembly.
To operate reliably, however, these pumps must stay within certain operating limits. Since they are a true positive displacement pump, discharge pressure theoretically spikes to infinity if the discharge is blocked. In reality, the motor stalls under these conditions, but usually not until after the flange bolts stretch or discharge piping bursts. Read More »

By Alex | Posted in Pumps, Technical Information, White Papers | Tagged , , , | Comments (6)

Mechanical vs. Solid State Pressure Switches

June 25, 2009 – 8:23 PM

Mechanical-Pressure-SwitchArticle written by David Gardellin of Onyx Valve Co

Onyx Isolator Rings have traditionally been supplied with mechanical gauges and mechanical switches to monitor process pressure and pump performance. These do a fine job, but many users are changing over to solid state switches. This paper will discuss some of the advantages of each.

Principle of operation:

Mechanical pressure switches operate with either a piston and spring, or a bourdon tube and mercury vial. In the piston and spring arrangement, when pressure builds up to the point where it crosses the set point, the piston pushes on a snap acting switch to transfer mechanical contacts.

In a mechanical bourdon tube switch, a bourdon tube tilts a rocking beam fitted with a glass vial of mercury. When pressure builds up to the set point, the beam rocks over and mercury in the glass tube rolls from one end to the other, where it submerges electrical contacts completing the circuit.

In contrast, a solid state switch uses a metal diaphragm about the size of a dime with a micro-miniature strain gauge etched onto its dry surface. Increasing pressure changes the impedance of the strain gauge. A digital comparator monitors the output from the strain gauge and compares it to the user specified set point. When the set point is reached, the electronic circuit turns on a solid state relay, completing the circuit.

Accuracy: A typical mechanical Bourdon tube pressure switch has a published repeatability of ±1% compared to the A-B 836 series which has a published repeatability of ±0.2%, a five-fold improvement in performance. Read More »

By Alex | Posted in Valves, White Papers | Tagged , | Comments (0)

The Effects of Capillary Tubing on Pressure Measurement

June 24, 2009 – 9:47 PM

Article written by David Gardellin of Onyx Valve Co

Capillary tubing introduces three effects on the performance of pressure-sensing instruments:

1. Temperature effects. Temperature changes cause the liquid inside the capillary tube to expand and contract, changing the volume of the fill fluid. The resulting error is a function of the total volume of the tubing, pressure instrument, and isolator ring. Because the rubber sleeve in the isolator ring has a much lower modulus of elasticity compared to a diaphragm seal, it can absorb most of the volumetric change resulting from temperature differences throughout the usable temperature range for isolator rings. A typical error in gauge reading through a temperature swing from 0°F to 120°F is about ½ psi depending on isolator ring size and gauge type. This is roughly a quarter the error expected with a standard 60 mm stainless steel diaphragm seal.

elevation-effects2. Elevation effects. As you change the elevation of the gauge with respect to the isolator ring, you introduce an elevation error. This error is due to the static pressure of the liquid in the capillary tube. The change in gauge reading caused by elevation changes of the pressure sensing instrument can be calculated in advance using the following equation:

P {psi} P Elevation{feet} sp gr actual gage = − ÷ 2.31∗

Our standard fill fluid is a silicone oil with a specific gravity = 0.967 at 77°F.

Observe polarity: If the gauge is above the isolator ring, then the elevation term in the above equation is positive; if the gauge is below the isolator ring, the elevation term is negative. If the gauge or transmitter has a zero adjust capability, the elevation error can be eliminated completely by re-setting the zero adjust to compensate for the elevation change. Read More »

By Alex | Posted in Valves, White Papers | Tagged , | Comments (0)

Become a Guest Poster and Get Published on the PetroPages Blog

June 23, 2009 – 1:35 AM
It's nice to be published

It's good to be published

If you would like to have your own post featured on this blog, write something of value to our audience. Keep the length to about 200 words and link to something; maybe the rest of a longer article, a relevant web page on your site, etc. Take a look at other posts here to see the style and kinds of topics that are appropriate. Email it to us— we will review it, approve it and post it.

Why would you want your information published on our blog?

  1. Branding—enhanced corporate image, greater exposure
  2. Sales—potential customers will be attracted to this blog and may lead directly to sales.
  3. Web Traffic—links back to your site and to your PetroPages listing will increase traffic to your sites.

Want to learn more about blogging?  Check out our post on the Basic Components of a Blog Post.

By Alex | Posted in PetroPages Updates | Tagged , | Comments (0)
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