How can a restriction ofice plate last with flowrate of natural gas of 550 sm3/hr, temp 41.7C & pressure 9.6 BARG? Is it 1, 2 years or 5 years based on experience so that we can schedule for inspection & maintenance? Or is it maintenance-free? The function of the orifice plate is to prevent overflow the natural gas dryer, which cause premature damages the desiccant inside.

Hullo Josh,

Clean Natural gas does not cause degradation. The pressure, flow volume etc are interesting but what causes degradtion is flow velocity, moisture content and particulate content. Assuming that the orifice plate material is suitable for the service, erosion is the principal failure mechanism.

How much damage is required before the consequences become significant? If consequences are sensitive to damage, it is probably best to inspect it visually at a convenient opportunity after a year or so in service and use that data to decide future inspections. If not, wait for the consequences, but keep a spare plate handy.

V.Narayan.

What would be the effect of moisture & particulate in the natural gas on instrument gas usres such as the wellhead control panels? How to know that the instrument failures are due to this? Rgds

Josh,

I am sure you know the answers! Particulates can cause shuttles and pistons in valves to get stuck, so you can expect maloperation. Liquid particles can cause viscous drag, with similar results. If you have the old flapper-nozzle controls, the holes can get blocked by partculates/moisture and result in falpper/nozzle failure.

Dew point and cleanliness standards are specified for these reasons.

V.Narayan.

The answers seem obvious to you Vee with so many years of experience. I'm still thinking & learning because I'm more on planning side. Your answers are practical so I'm going to look for evidence of these in the instrument gas consumers if they are easily visible at site. Sorry, what are the old flapper-nozzle controls, what is the new control? Just to explore this subject before asking the instrument engineers.

Do you know a good brand of a portable dew point meter (non-bulky & hand-held)with sampling cell? I got a Vaisala brand but not sure whether it's going to work. Filters specified to be 1um. Rgds

Hullo Josh,

In pneumatic systems, the flapper/nozzle control devices were common 30 or 40 years ago and are still quite prevalent. I am recalling things from over 30 years ago, so I can be mistaken in what I say below; take it with a pinch of salt.

The flapper is a paddle like strip that, in its natural state rests against the orifice of a nozzle. If you blow air (or gas) through the nozzle, it pushes back the flapper. If you control the movement of the flapper by your signal, say by a bourdon tube, the back pressure on the nozzle will vary accordingly, depending on how far the flapper is away from the nozzle. This variation is fairly linear over a 4-16 mm H2O pressure range, so this back pressure reading tells you the signal strength.

The nozzle is quite small bore. I cant recall exactly, but I think it is 0.3mm or less in bore. Instrument engineers on the forum can correct me on all of these points, but my explanation is not relevant to your original question - why do particulates and moisture matter? You should refer to a reference book on instrumentation from your library to get details.

Pneumatic control ssytems have a number of small bore valves, shuttles, pistons and cylinders that operate best on dry clean air.

I am not very familiar with current designs of dew point instruments. A Google search will give you options and others on this forum are more experienced with them.

V.Narayan.

I have searched a bit about the instrument activating devices which are hi-lo pilots, pilot relays, 3-way valves, solenoid valves etc.

What would be the maximum tolerable ID loss for the orifice? The ID was machined to +-0.25mm. Is it ok if I specify the max ID loss of 0.25mm for replacement of the orifice?

Josh,

There are flow measurement orifice plates for which your Instrument dept. will have the atandards for rejection. API and other organizations publish the requirements, but I cant recall the Ref. Nos.

We also use orifice plates as restriction devices, where we can be more lenient.

The +/- 0.25 mm is a manufacturing tolerance I think, not one for measuring wear.

There are many published stds. so do a trawl; talk to your instrument people.

V.Narayan.