4 replies to this topic

[R NESAMANI]

Dear all,

 

We have to supply a natural gas flow from pipeline1 to pipeline2. Pipeline1 operates at 80 kg/cm2 & Pipeline2 operates at 72 kg/cm2. Consumption in pipeline2 is 50000 scm/hr.10000 scm/hr is provided from other source. so we have to supply 40000 scm/hr. We think to install a Orifice plate for reducing pressure & supply 40000 scm/hr (or) 550 Cu.M/Hr at 80 kg/cm2 & 35 degC . Orifice plate diameter calculated as 1.475 cm .

Inputs: Q = 550 Cu.M/Hr ; D1 = 18 inch with 15.9 mm thick PL ; Cd = 0.6

 

1) Kindly check my calculation for orifice diameter 1.475 cm correct or not?

2) Can we just drill a hole(1.475 cm dia) in spectacle blind and use this blind as orifice? Whether this will solve my purpose?

 

Kindly guide me.

[Art Montemayor]

 

What are the pressure ratings on both pipelines (P/Ls)?  Since only 8 kg/cm2 (113 psi) separates both lines’ operating pressure, it would appear that the pressure ratings for both P/Ls are equal.  If that is truly the case, then why not just connect both lines directly and allow them to operate at the desired pressure that you need to furnish your demand downstream?

 

Please furnish more information (such as your missing orifice calculations and scope of work) as to what is limiting your ability to raise the pressure in P/L #2.  All gas pipelines are subject to the discharge conditions of P/L compressors and 8 kg/cm2 is not a significant additional pressure to impose on a natural gas transport P/L.

 

A transport pipeline does not normally work on the principle of steady state flow.  The downstream consumptions are subject to variation (such as peak loadings) and you normally cannot rely on an orifice to supply a steady state of flow rate.  You require a device that can be adjusted to the varying demand and keep the downstream pressure constant (or almost constant).  That device would be a pressure control valve.  The “line pack” in a P/L is what functions to lessen the effects of a variable pressure in the P/L.  The goal of a gas compression station is to maintain a constant pressure in the P/L, not necessarily a constant flow into the P/L.

[Neelakantan]

@nesamani

 

now you have moved from using a globe valve ( http://www.cheresour...ch-globe-valve/ ) to using an orifice plate?

 

in both posts you are giving piece-meal information, yet we are responding to this query for the reason that we consider that there are essential safety concerns in such a situation.

 

Note that ART and I have stressed both times about the working and design pressure of both lines. also additional information such as line size - dia, route profile and length-, instrumentation, packing and depacking operation modes, shutdown concerns, etc are all required for a proper engineering solution

 

as ART mentioned, an orifice plate is a pressure reducing unit ONLY AS LONG AS there is a continuous flow; that is there is steady withdrawal from the second line; when there is reduction or stoppage of flow the second line will get packed and pressurized.

 

 So when we get more, precise engineering information then, we can give a PROPER, WORKABLE and SAFE engineering solution.

 

regards

neelakantan

Edited by Neelakantan, 07 July 2014 - 01:21 AM.

[R NESAMANI]

Pipeline1 is 1500 km long cross country pipeline with customers. Pipeline2 is 1000 km long cross country pipeline with customers.

Both pipelines design pressures are 92 kg/cm2. Pipeline2 is new pipeline with very few customers, capacity is 16 mmscmd, but consumption by customers is only 1.2 mmscmd. So we want to operate this pl at low pressure i.e 72 kg/cm2.

 

Requirement is to maintain the pressure of pipeline2 at 72, if supply to pipeline2 from pipeline1 stopped nothing will happen, as line pack is available in pipeline2 for some days consumption. Customers requirement is 40 kg/cm2 maximum.

 

By installing a globe valve, we can control & supply the flow required. The procurement is in progress.

But my view is to design & install the orifice to supply gas to pipeline2 for temporarily. 

 

Inputs for orifice design is Q = 550 Cu.M/Hr {40000 scm/hr at 80 kg/cm2 & 35 degC}; D1 = 18 inch with 15.9 mm thick PL ; Cd = 0.6, P1 = 80 kg/cm2;  P2=72 kg/cm2; density of natural gas = 0.7 kg/cm3 ; This gave me output as required dia of orifice = 1.475 cm. Is it right?

 

Also making a hole of this dia in blind flange can act as the orifice or not?

[Neelakantan]

with both lines with same design pressure, i still do not comprehend the reason for reduced pressure that you intend to "maintain"; as ART mentioned, both lines can be pressure-floating and the consumers on line-2 will decide the flow from first to second line.

 

btw, are they GAIL lines or RLNG lines? where are they located?

 

regards

neelakantan