OTR Tanker Offload Rate?

[blackwed]

What does experience say is a good rate to expect to get when offloading jet fuel from a tank truck? Most everyone I've talked to agrees on about a 325+/- gpm max rate for a gravity drop. Can the rate be improved appreciably with a pump or will that just vaporize the product (RVP <0.1 psia)?

DB

 

[TD2K]

What size connections are these? I would guess for this rate they are 3"?

A pump will speed things up. You won't start to cavitate the pump till the pump suction pressure is well below atmospheric because of the low vapor pressure of jet fuel.

When you are doing an atmospheric drop you have the static head to consider, say up to 12' when the tank truck is full and maybe a few feet at the end. So the dP across the hose (taking grade as the elevation the nozzle is discharging at) is between 4.5 psi to maybe 1 psi as the truck drains. If you have a pump on it, you can increase the dP across this suction hose significantly to maybe as much as 3x when the truck is full (assuming a minimum suction pressure at the pump suction of -12 psia) which should give you about 70% more flow. You can work through the numbers at the end of the unloading period to estimate them compared to a gravity drop.

Rough estimate with a pump, at least 50% more flow. I would think 100% more flow would be the upper end. If you need more than that, you need a bigger hose.

 

[blackwed]

Thanks for the information, TD. Would it matter whether a centrifugal pump or a PD pump were used? The pump in use is a PD.

DB

 

[jmw]

Is there a velocity standard involved? i.e. to limit static electricity generation in the vessel to which the fuel is discharged?
I think there was one for hydrocarbons generally but can't be sure.

JMW

http://www.ViscoAnalyser.com

 

[blackwed]

Any issues relating to static discharge are outside the scope of my problem/question. I just need a quick trip to pump school as I have two very experienced (and heretofore valued) engineering contractors giving me advice that's 180 deg apart.

Thanks,
DB

 

[CRG]

JMW, a prudent engineer would/should consider some of design criteria that are discussed in API RP 2003 regarding static electricity, fluid velocities, etc. The issue may be outside the scope of backwed's question; however, I doubt that the concern should be outside of the scope of the process.

 

[blackwed]

Myriad questions can be raised about any facility/project/system/process. As a prudent P.E., I will not ignore any that are pertinent to any of my work and welcome anyone pointing out those I may not have considered. However, my question today is how fast can one reasonably expect to get jet fuel out of a tank truck and how two different types of pumps would perform. I don't need to know how to make a watch right now, just what time it is.

Thanks,
Dan Blackwell

 

[Montemayor]

I think you'll be unable to obtain 325 gpm with a 3" suction on a centrifugal pump. I would always use a centrifugal pump on jet fuel. A gear pump (or a positive displacement pump) will contribute metal filings or friction products in the fluid - something I won't tolerate in jet fuel. Also, the fluid does not have much "lubricity" and the friction is high with a metal-to-metal displacement. A centrifugal pump will meet most or all transfer requirements.

I don't know what you plan to use as a diameter for the pipe on a 325 gpm gravity drop, but it better be a formidable size. Much bigger than 3" and more like 8 or 10" if it has to be self-venting. TD2K is exactly right; you're going to need a goodly hydrostatic head. The centrifugal pump is much more reliable and quicker.

 

[MikeHalloran]

I'm curious about why you're in such a hurry to empty the truck.

And whether you've accounted for the time it takes to park the truck, chock it, ground it, wrestle the hose into position and connect it, dip the tank, dip the truck, calculate the available volume in the receiving tank and the volume to be dumped from the truck, survey the area for ignition sources, and open the valve.





Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[rmw]

Mike,

You forgot to mention 'go to the restroom, and stop for a cup of coffee on the way back to the truck' before commencing the details you list.

rmw

 

[Montemayor]

Mike & rmw:

I'm with you guys in wondering why an increase in unloading rates on a tanker-truck is so important. I've always designed for a maximum of 150 gpm unloading rate on rail and truck tankers and found that considering what other duties have to be done related to the chore (especially the safety precautions and steps) the rate is not that important in saving time. The % time element that the actual fluid removal represents is one of the smallest of the total time required for the procedure. For example, Mike left out the very important grounding procedure; but I know he intended to include this - and others - to make his point.

In unloading liquids - especially jet fuel - the rate of removal is not as important as making positively sure that all the safety steps and precautions are checked and double-checked. All it takes is one mistake, after the pump-out is activated, to provoke a major incident. I've had product pumped into the wrong tank! I've investigated an incident where a tired truck driver connected a compressed, pure Hydrogen hose into an Oxygen manifold in a Canton, Ohio plant in 1961. The result was, of course, disastrous. I'm not relating this to scare or impress other experienced engineers. We are all aware of the human frailties we have to consider when we design an operation. The speed with which one can unload is an attractive feature; however, when viewed from the human aspect of other duties being required of the same individual and involving a hazardous, combustible fluid, I give it second priority to making dead-certain that all safety procedures are 100% in place and the operation is routinely safe and successful - albeit, slower.

However, I may have the wrong idea about this operation and it may be similar to loading the Space Shuttle with its emphasis on speed. If so, then there are trade-offs to pay.

 

[jmw]

Dan,
appologies if you felt my query unhelpful but while it is presumed there are no liabilities incurred by contributors to these fora, some members feel it as much a responsibility to point out possibly relevant issues as they do to not give bad and possibly dangerous advice.

Similarly, some members like to have addittional, if not directly relevant information, so that they can better understand the application which may in turn lead to more pertinent aadvice being given. It also pays never to assume facts which are not in evidence.

Plus some of us are just plain nosy, it's a small price to pay, isn't it?

Please bear with us if we do introduce comments that may not be directly responsive to your initial question. I personally find that such comments can be very helpful. I don't know what the other members feel about this.

Perhaps you would share the opposed views of your contractors. If we assume you have all the other bases ready for when you build your watch, knowing a bit about wher the contractors disagree may lead to some more pertinent comments from some of the other members.

By the way, is this a bridging tanker delivering to site fuel storage or for fueling planes? are you unloading over a weigh bridge or through flowmeters? Any increase in pump rate will also need to be reflected in the flowmeters, if used (either on the truck or in the receiving installation), and flowmeter dP may not be linear with flow; the pump will have to cope with the increased headloss in the delivery system.

JMW

http://www.ViscoAnalyser.com

 

[blackwed]

I've been reading many of the Eng-Tips boards for several years and have always found useful, accurate and appropriate information along with professional attitudes the norm. Sorry for my attitude last week. It was one of "those" Friday afternoons. I do appreciate everyone taking time to read and comment on my post.

I have a site at which I have been tasked to manage a project to provide temperature compensated metering in the jet fuel offload truck rack. The facility has been in operation for years and the scope of this project is to accurately meter the offloads.

The existing LC M-60 meter (which is largely ignored currently due to air in the fuel stream) will be outfitted with a temperature compensator and an air eliminator will be installed--somewhere.

The facility has a 400 gpm PD pump with 6" piping between it and the hose that is currently used for offloading. The story I get is that the old centrifugal pump was too slow.

The fuel is pumped into bulk storage, several steps and several days ahead of introduction into an aircraft.

Leaving out all the "betcha's" and "ain't-no-ways", I'm left to chose whose advice to take--the contractor who says that the PD pump will never actually move 400 gpm of product from a truck because pump suction will vaporize the product if you attempt to move the fuel appreciably faster than the rate gravity will support or the one that says not so and that even higher rates should be possible in a properly designed system. Both contractors have been engineering, installing and operating a wide variety of fuel facilities for probably 30 years, and until now I have had complete confidence in almost every technical or operational detail either has been asked about.

Contractor #1 wants to re-gear the pump down to 300 gpm and install an air eliminator of his own design (vertical vessel) in the pump suction line. It'll have switches to control pump start-stop when the level gets low and valves to control venting air.

The site operator says we can't do anything to reduce his offload rate to that. He's wanting us to put the AE vessel in the pump discharge line ahead of the meter. My ktr says that will require a flow control valve downstream of the meter so air can be expelled from the hose and suction piping before the meter sees any flow and neither he nor I find comfort in that potential blockage downstream of the PD. It also does nothing to keep air out of the pump.

Both contractors and I agree with above posts (including RMW's) that there are so many other tasks involved in completing the offload process that the additional few minutes of pumping time introduced by limiting the rate to something 300-ish is meaningless. However, the operator "has to get the trucks out as fast as he possibly can." Orders from above are to do nothing that will reduce what he says he has to have.

Can I expect to safely and accurately meter fuel taken from a tanker using a 400 gpm PD pump?

Dan

 

[Montemayor]

Dan:

I don’t speak for all the talented individuals that contribute help on the Forum, but I can assure you we do it because we think we can help out in a given situation we are familiar or experienced in. Like my colleagues, I never take anything related to solving a problem – whether critiques or complaints – personally. The important thing is solving the problem – not who can come up with the best ego trip or show. I believe everyone recognizes your problem as an important one and desires to help out.

Your additional basic data and scope of work really helps us out in furnishing expertise and comments because some of us have done this (or something similar) before. I don’t know where the included (or entrained) air is coming from or how it gets in the jet fuel (JP-1?). I can only assume the air is aspirated or entrained with the Jet fuel when the level in the tank truck is low or reaching the empty mark at the end of the unloading. Is this correct? I designed and installed Helicopter JP-1 fuel facilities and never had to allow for air inclusion in the fuel.

I’ve found unloading a tank truck to be a piece of cake. The critical design criteria for this chore are:
1. The maximum unloading flow rate imposed on the system;
2. The capacity and characteristics of the unloading pump;
3. The size and type of suction connection to the unloading pump (is pump truck-mounted or part of storage tank system?);
4. The NPSHr of the unloading pump (this is really critical and must be correctly known).

It’s also important to know how you are “dumping” the fuel into the storage tank: is it through a bottom nozzle or is it through a roof-mounted dip pipe? Either way, you need some minimal instrumentation.

Pumping 400 gpm of jet fuel through a 6” suction should be very easy assuming:
1. your unloading pump suction line is sized adequately to conform to the pump’s NPSHr
2. your unloading pump conforms to the total developed head of the system.

Someone has to do the hydraulic and NPSHa calculations for your proposed unloading system. And these hydraulic results must conform to the pump and the system’s characteristics. There is no getting by without this being done – this is a professional engineering minimum pre-requisite. Whether you select a PD or a centrifugal type of unloading pump is up to you or whoever is in a decision position. However, the pump must conform to the NPSHa and the system. Otherwise, it “ain’t gonna work right – or at all.” The size and the capacity of the suction line is analogous to an electrical conductor, except that the electrical conductor allows a reduced amount of electrical current to pass. A deficient suction line design will not allow the unloading pump to pump jet fuel if the NPSHr is not supplied at the “eye” of the pump. It won’t work, period. In fact, design criteria calls for 1.35 X the NPSHr as the design figure to supply a pump – especially one with a relatively high vapor pressure, such as jet fuel.

It may be true that your PD pump will not pump the 400 gpm. We can’t tell whether the statement is true or not without the hydraulic and NPSHa calculations. And yes, you can pump as high a rate as you want to – if you supply the appropriate pump and system. That’s not a tough engineering problem. You simply get a pump with a “good” NPSHr and make the piping large enough to yield a low pressure drop on the suction line and the discharge as well. But then, you’ve got to generate the hydraulic and NPSHa calculations. If your contractors haven’t furnished you with their version of the hydraulic and NPSHa calculations, I wouldn’t believe a word they say. No professional engineer installs this system without doing the required and necessary hydraulic and NPSHa calculations because they are so easy and without them, any recommendation is not credible.

If I were given the opportunity I can bet my year’s salary that I’ll pump 400, and even up to 1,000 gpm if I have to – and am legally allowed to do so. That is no problema. The problemas start appearing when the contractor is told he can’t increase the size of the truck’s outlet nozzle, he can’t use the properly designed pump, he can’t increase the size of the suction line, and he can’t increase the size of the system. I’m assuming you’ve got tank trucks with approximately 5,700 gallons capacity each.

I would not use a gear or PD pump on this application unless forced to do so. A centrifugal will do this job with no metal-to-metal contact and much less internal maintenance. The total system developed head will be a very small amount and you certainly do not need the high discharge pressure capabilities of the PD. Additionally the footprint of the PD will far outstrip the space and size of the centrifugal. A PD is a vast overkill and a reduction in pumping capacity. However, you can certainly employ a PD and, if designed and hydraulically calculated correctly, it should operate and do the job safely.

 

[jmw]

Hi Dan,
that's fine.
Let us know when you want to build the watch, there are some comments and questions I have regarding this, especially the temperature compensation.

By the way, it is my understanding that most air problems are due to starting with empty hose and fixed pipe prior to the meter. An air release unit should fix that and LC make these also. I don't think it strange that one contractor wants to design his own, Neptune's French distributor didn't use the Neptune standard and designed their own which Neptune then used for some applications i.e. he could have good reaons for this.

The other source of air is through leaking glands at pump suction or vapour breakout at high pressure drop.
This is best prevented by good maintenance and system design as it is more difficult to exclude using an air release unit which are not 100% efficient. Kept to a minimum the impact on volume measurement is manageable.


JMW

http://www.ViscoAnalyser.com

 

[blackwed]

1. Tankers are commercial 8000 gal bottom loaders. I believe the onboard piping is 4" but may be 3".
2. Product is JP-8.
3. Sources of air are 20' of empty hose/line at startup and aspiration starting at about the 40-50% level in the tankers.
4. Existing PD is Blackmer GX-4B geared for 420 rpm.
5. During one operation, observed pressure d/s of the existing meter was appx 35 psi. This is maybe 20' of 4" piping away from the pump discharge with a check, a plug and three elbows in between and is maybe 1000' u/s of the bulk AST's.

I would appreciate any thoughts you have regarding temperature compensation as this is the reason for this project. Also insights into vapor breakout and under what general conditions in a system such as this one it might occur would be appreciated.

I think my contractor wants to use his AE design because he feels it's a good one. I tend to agree. The vessel doesn't have to be coded since it's in the pump suction line and due to the location it not only eliminates air from the meter but from the pump as well. It makes everone happy except the operator who says he cannot live with the gravity drop offload rate that it imposes.

Much like when I converted my 35 year old truck to power disc brakes, I am growing weary of this project well before its completion.

Dan

 

[Montemayor]

Dan:

Now, with additional basic data, I can tell more accurately what you are facing. I'm also assuming you're not free to make any changes in the tanker outlet nozzles, the suction piping, or the discharge piping. With these caveats, I estimate you'll get a maximum of 125-150 gpm out of your tanker truck (assuming you have a generous fluid static head over the pump. All the wishing and designing for 450 gpm is just wishes; you'll never get there with what you have described. It'll have to be modified if you are to achieve 325 or 400 gpm.

The controlling offloading capacity factor is your pump's NPSHr and the available NPSHa that you give it. In case you are not familiar with hydraulics (you don't make any mention of any hydraulic basic data), the NPSHa of your system is determined by the jet fuel's flowrate, the diameter of the suction nozzle and piping, the height of the jet fuel's level above the pump, and the vapor pressure of the jet fuel. I am assuming that the tanker truck is vented to atmospheric pressure and not blanketed with an inert gas, like Nitrogen. I normally vent and displace all air in the suction and discharge piping by simple, 3/4" or 1" vent lines recycled back to the tanker truck and I never have any air inclusion to worry about in the actual liquid off-loading.

If you can slow down the Blackmer gear pump (or reduce it's capacity), you should be able to unload Ok - but at about 125-150 gpm, as I estimate.

Hope this helps out.
(One personal comment: you're not going to successfully design this unloading operation without a complete hydraulic calculation. That's not me insisting on this; that's common sense. You can't engineer yourself around natural engineering laws, and the only way you can predict what to expect and plan for as unloading rates and times is to generate a hydraulic calculation as I outlined.)

 

[blackwed]

I'm hydraulically literate only to the point that I know what npsha & ...r and a few other terms like them mean but not to the point that I would confidently make any calculations, certainly none I would present. I am no more at home in your discipline where one can comprehend a parameter that has units of psi-sec, than in one where in^4 has meaning.

So, based on your estimates, what all's possibly going on now inside or at the inlet of my guy's pump running at 400 gpm?

Dan

 

[jte]

blackwed-

You may not be at home in the world of psi-sec or in^4... but I've always wondered about folks who run their current calc's backwards (don't the electrons actually run neg to pos?) and use imaginary numbers on a regular basis!

jt

 

[Montemayor]

Dan:

I’ve followed exactly what you have stated and expounded:

1. “my question today is how fast can one reasonably expect to get jet fuel out of a tank truck and how two different types of pumps would perform. I don't need to know how to make a watch right now, just what time it is.”

2. “I just need a quick trip to pump school as I have two very experienced (and heretofore valued) engineering contractors giving me advice”.

My comments have been directed at the problem, regardless of your aptitude or qualifications. I’ve assumed you are managing this project and that you are the key individual who will also manage the contractors and their results. If you don’t commission your contractor’s (or someone else) to furnish a competent hydraulic study and NPSHa confirmation, that’s your decision to make. I’ve managed projects up to the multi-million $, grass-roots level and I’m very much aware of what a manager has to do to get a job done. You and I both know that the manager’s knowledge or lack of knowledge in the specific problem at hand is not a central issue. Getting the correct, safe, and efficient solution is. And all engineers – regardless of discipline – are (or should be) trained to carry out this function. That’s what P.E. exams are all about.

If your present gear pump is running at 400 gpm rate and using a 3” suction line size to do it, I can bet my bottom buck that it is gasifying the jet fuel in the suction and pumping at a strained and crippled flowrate. The fluid may cavitate further on, after gasifying, but the major effect is that the flow rate is affected in a PD pump. This would explain the "included air" effects noted and any metering problems. It can get so bad that the gasification will make the pump loose prime, which means zero gpm of flowrate. That is no way to run a railroad, and much less to design and operate an offloading facility for a combustible and hazardous fluid. Without the required basic data I’ve already detailed (suction line size, length, static height, etc.) we can’t even calculate a simple, estimated NPSHa to match against your pump’s NPSHr – which also hasn’t been identified. These are all important items that have to be identified and confirmed in order to ensure a proper and engineered off-loading project.

The main concern I would have is the properly designed and operated off-loading with accurate, predictable results. I am not criticizing your contractors or the way they are going about the project. As an engineering manager I expect the contractors to be 100% under the control and direction of the Project Manager. They should be doing what they are directed to do.

My above comments are all given in the interests of adding engineering value and positive results to your efforts. I hope you take them into consideration.

Good Luck.