allowable velocity criteria

[patibar]

What is the maximum allowable velocity for a nitrogen service line which is an intermediate service (not continuous)?

 

[zdas04]

Do you mean "intermittent" instead of "intermediate"?

I hope I'm wrong, but I don't think you are going to get a definitive answer to that question. The only published maximum velocity that I see is from API which is an empirical equation in U.S. units (100/(rho)^0.5) with rho in lbm/ft^3 and the answer in ft/sec. That gives reasonable answers for liquids, not so good for gas (I'm assuming here, that your line is gaseous nitrogen, not liquid), I'm not sure why I'm making that assumption).

Different companies have different steady-state velocities that they state for a maximum. Basically in gas maximum velocity is an economic parameter (the faster the gas goes, the bigger the pressure drop to friction that must be replaced at subsequent compressors() not a physically-limiting one. This is a little confusing so let me give you an example--the laws of physics limit the flow from high pressure into low pressure to the speed of sound (unless extraordinary piping configurations exist) so the speed of sound is a physically limiting parameter. If a company says that you can't design a flow line with an average velocity greater than 30 m/s that is an economic consideration and if your gas went 35 m/s nothing particularly bad would happen.

Now for intermittent flow you have a whole new set of rules. Let us assume that between pressurizations the downstream line is vented to atmosphere and that the line upstream of the pressurization valve is at a pressure greater than 2 barg. When the valve first opens the velocity through the seat will be Mach 1.0. You don't have a choice about that. Mass flow rate is under your control, but velocity isn't (if you are not clear on choked flow see faq378-1201). After a time the pressure in the line will increase until the flow is no longer choked and the velocity will decrease (and you will begin to be able to control it).

If the downstream pressure is above the critical pressure for choked flow at the beginning of the process and you want a maximum velocity I'd use 30 m/s because that is a number that many people have adopted and you probably won't have to spend much time explaining it.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[TD2K]

I don't usually worry about dry gas velocities in terms of a maximum. PSV discharges as one example can get close to Mach 1.

As David has said, at higher velocities pressure drop can quickly become the limiting factor in how much you can flow down a header.

 

[BigInch]

Noise can get pretty loud over 100 fps.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[bcd]

Lots of things to consider when selecting maximum velocities. Here is some data I use:

Recommended flow velocities for liquids:

Typical piping design ranges from 2 to 3 m/s (6 to 10 fps). Inlet velocitites into valves can range from 7 to 12 m/s (23 to 39 fps) depending upon the type of valve. These values are for clean liquids. Any impurities require lower velocities. Other factors to consider include piping pressure losses, corrosion due to wearing away protective films from metal surfaces, and high inlet velocities can cause instability in control valve trim. Cavitation and flashing risks increase as flow velocity increases.


Recommended flow velocities for gas:

For continuous duty, Vmax less than 0.5 Sound Velocity
For intermittent occasions such as gas to flare, Vmax less than 0.7 Sound Velocity.

Again, above values are based on clean gas streams. Any impurities require lower numbers. Also need to pay careful attention to noise levels. Common industrial limit is 85 dBA for worker exposure. Common limit for mechanical equipment is 110 dBA because small, lightweight mechanical and electrical equipment starts to break down due to vibration.

 

[BigInch]

0.5 Mach speed! That's way far too much continuous noise.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[maddocks]

We'll go as high as 0.7 mach for short lines and intermittent or emergency flow scenario's. Reduce to 0.50 mach if you're nervous - all this is assuming you have pressure to spare. If noise is a real serious consideration, then go back to conventional line sizing criteria of 1-2 psi/100'.

 

[zdas04]

53-106 psi/mile????? Have you never been responsible for economic success of a flow line? The most common numbers I see are 2-5 psi/mile for lines bigger than 12 inch and 15 psi/mile for smaller lines. If you were going from New Mexico to the LA Basin and you could only start at 1200 psig and would only allow the pressure to bleed off to 800 psig, then you would need a booster station every 4-8 miles at 1-2 psi/100 ft. Around 100 booster stations would be needed. In the actual design there tends to be 3 stations.

As I said above, most of the companies I work with cap velocities somewhere around 100 ft/sec. 0.5 Mach would give you a number somewhere around 450 ft/sec. Oh, yeah that would put you squarely in the transsonic region of the velocity vs. compressibility curve so none of the incompressible-flow equations would give you meaningful results (i.e., it would not be possible to predict pressure drops using something like the AGA Fully Turbulent equation) and the compressible flow equations tend to blow up with gases slower than 1.0 Mach.

If you are really designing flare headers with design velocities of 0.7 Mach, I hope you are leaving a LOT of safety factor in your PSV sizing. I designed one the other day and was a bit uncomfortable at 120 ft/sec (around 0.09 Mach for this gas at predicted temperatures).

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[maddocks]

I assumed that this is in-plant piping. Pressure drop criteria for in-plant piping is anywhere from 1-2 Psi/100' - has been for the the 30 years I've been doing this. So, yes, I think we can assume that I have been responsible for some minor economics.... If it's a pipeline, then default to the 15 psi/mile. As far as mach number, it's all about pressure and the resulting effect on the system.

 

[zdas04]

I've never worked inside of plants thank goodness. Most of my clients right now are plant people that I'm trying to help build field stuff without hanging a ton of plant baggage on it. Funny though, they all have extensive standards manuals that all limit process piping for gases to some number around 30 m/s or 100 ft/sec (I know that those are not the same number but I get a strong feeling they have the same origin). I've worked for 7 of these "plant" companies and all of them have a max process velocity around 100 ft/sec in their standards. Maybe number 8 will be different. I had never come across 1-2 psi/100 ft until your post.

As to the 15 psi/mile number that comes from slide rule days when you tried to come up with a "normal" value for as many parameters as possible. If "normal" downstream pressure was 100 psig, then 15 psi/mile was about 60 ft/sec, at 500 psig it is 23 ft/sec, and at 20 psig downstream it was 100 ft/sec. Someone back in the day felt that these numbers made economic sense. The stuck. I have looked for a technical basis for the 15 psi/mile (or the 100 ft/sec) for years and never found a hint of where those numbers came from.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[patibar]

Nitrogen is available at 9bar(A)and ambient temperature. We use nitrogen for purging of an Hydrocarbon line and vented to atmosphere. So DP is 8 Kg/cm², expected choke flow condition. The frequency of this activity is twice in a month. What is the maximum flow i can provide for this by considering maximum allowable velocity?

 

[zdas04]

If you are trying to inert a line either through a clearing purge or a dilution purge, then you just don't need that much pressure upstream. For a clearing purge you would want a regulator set somewhere around 1 bar(g). That will still be at sonic velocity, but dropping the upstream pressure reduces the mass flow rate into the process so you don't throw as much nitrogen away.

For a dilution purge, you only have to raise the pressure in the line to about 0.5 bar(g) to reduce the amount of oxygen to levels that will not support combustion. I would do this with a pressure regulator set at something like 0.7 bar(g) followed by a choke nipple with a pretty small hole through it.

For a twice monthly evolution you really don't need to worry about velocity as such (although you may very well have to limit velocity to reduce noise). Momentum effects of high-velocity gas are acceptably small, and errosive velocities occur at multiples of the speed of sound.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[BigInch]

Large diameter Gas transmission pipeline economic velocities usually default to 20-30 fps. 40 fps gets rough on compressor station spacing.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[zdas04]

At a nominal 1000 psig downstream pressure, 20 fps is 25 psi/mile. 40 fps is 80 psi/mile. The numbers I see on pipelines out of the San Juan Basin are closer to 3 psi/mile (around 7 fps).

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[BigInch]

Right. That doesn't make sense. I meant in the context of maximum velocities. Default wasn't the right term. It's too close to meaning average. Compressor station spacing in the context of, if you reach those velocities anywhere, the pressure is so low that you need to put a compressor station well before that point.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek