Process Engineer

Critical - Subcritical

2011-03-24T17:03:00.032+07:00

Hi my friend, how are you today? I am sorry, It’s been a while my blog has not been updated. I was very busy this week; therefore I had no time to write a new article. Ok, Let's continue the discussion about back pressure. Now, by this article I will explain you that the critical and subcritical condition depend on the back pressure.

Before I make further explanation, please note, explanation in this article will be limited for gas vapor phase application only.

API 520 part 1 provides calculation procedure for sizing of effective area. The procedure is divided to critical and subcritical condition that depends on the back pressure that exists at outlet PSV. Critical condition is considered when the back pressure is lower than the critical flow pressure.

Critical Flow Rate

How critical condition occurred? If a compressible gas is expanded across a nozzle or orifice, at constant upstream condition, its velocity increases with the decreasing downstream pressure. The increasing velocity means that the mass flow rate increases. The flow rate will increase until a limiting value.

At sonic velocity, the flow rate could never increase anymore even though the downstream pressure is much lower. This maximum flow rate at sonic velocity is known as critical flow rate.

see this picture.

Critical Flow Pressure

To avoid confusion, terminology of critical flow pressure is used instead of critical pressure. Critical flow pressure is defined as an absolute pressure at nozzle exit at critical flow rate. The actual pressure at nozzle exit cannot fall below the critical flow pressure even though the downstream pressure is much lower

Critical flow pressure can be calculated using the ideal gas relationship.

See above figure. For PSV, pressure at outlet is known as back pressure. Based on the back pressure value, we can determine whether the condition is critical or subcritical.

Critical condition is considered if the back pressure is less than the critical flow pressure. In other word, if the back pressure is greater than the critical flow pressure, it will be considered as subcritical condition.

The orifice calculation procedures both for critical and sub critical condition shall be used in each appropriate condition.

My friend, that’s all, I can share to you. Critical and subcritical condition depend on the back pressure. Hopefully, this topic reminds us that the back pressure is something very important to be considered when we conduct PSV sizing.

Thank you for your attention.

Back Pressure Effect

2011-03-11T08:31:00.014+07:00

In the previous article, two kinds of back pressure have been discussed. We have already understood that back pressure has several impacts to PSV performance. In this article, we will learn the impact of back pressure on the relief valve opening, operation and flow capacity.

My friend, If you don’t have a good understanding on the definition of terminology such as; overpressure, accumulation, set pressure, re seat pressure etc, I guest you will be confused with my explanation. In this article, I won’t to explain each of them, so please refer to API 520 part I by your self. It’s very important to understand the meaning of its definition.

PRESSURE RELIEF VALVE OPENING

Superimposed back pressure has impact to opening of conventional relief valve type. This back pressure will give additional spring force onto valve disk in closed position. Therefore, the actual spring setting can be reduced by an amount equal to the amount of superimposed back pressure.

Hopefully, this picture will help us.

For balance and pilot type, the compensation of the constant superimposed back pressure is not required

PRESSURE RELIEF VALVE OPERATION

Excessive of built up back pressure has impact the conventional valve operates in unstable condition. Its may be chatter or flutter. Chatter is rapid motion of closing and opening valve where the disc contacts with the relief valve seat during cycling, whereas flutter is not contact with the seat. Chatter cause damage to the valve.

PRESSURE RELIEF VALVE CAPACITY

Built up back pressure has impact reducing the valve capacity. High back pressure reduces the lifting of disc result in reduction of flow capacity. For conventional type, built up back pressure shall not exceed 10% of set pressure at 10% allowable overpressure. For application that allowable overpressure is higher than 10%, say 16% of multiple valve application, then the built up back pressure up to 16% of set pressure is allowed for conventional type.

See the picture below of capacity correction factor due to back pressure for conventional type where the spring setting compensation for superimposed back pressure is required.

The figure below shows that the capacity of balance below type will be reduced significantly due to back pressure. Look at this picture, hopefully you get the answer why we choose to use balance below type for back pressure up to 30% at 10% allowable overpressure.

Finally, I can share this topic for you. Hopefully we get better understanding on the importance of the back pressure. The back pressure is one of consideration for optimization flare system. For new design system, which one do you prefer, use larger size of tail pipe to reduce back pressure or use balance below type to overcome it?

Friends, Thank you for reading. Please correct me if I am wrong.

Built Up and Superimpossed Back Pressure

2011-02-28T13:50:00.020+07:00

Back pressure is defined as pressure that exists at PSV’s outlet. It has impact to performance of PSV, hence its value should be informed to vendor therefore shall be stated in the datasheet. Good concept understanding of the back pressure is helpful when calculates its value.

In this article, the following term is used for easier understanding
1. PSV in Ready Mode - PSV still in closed position - system pressure less than PSV’s set pressure - so that the PSV is not open yet
2. PSV in Open Mode - PSV in open position - system pressure higher than PSV’s set pressure - so that PSV is open

Superimposed back pressure is pressure coming from other sources when PSV in ready mode. It may be constant or variable depends on the sources.
Built up back pressure is pressure generated as a result of fluid flowing through PSV in open mode.
I hope this picture will give you better explanation

The picture above seems very theoretical. I do very understood that you prefer something more practical.
See this picture. It will help you get better understanding of the constant back pressure.

When blocked outlet case at downstream occurred, the PSV will open and built up back pressure is generated. See this picture.

The other important point is the spring PSV shall be capable for handling pressure not more than 7.5 psig, so that PSV will start to open at 10 psig. For this case, let your vendor know with the system and back pressure value.

Here is the other system has variable superimposed back pressure.

Back pressure can be calculated by means of hydraulic calculation, Flarenet or HYSYS simulator. I suggest you to use FlareNet for project and critical system where accurate calculation is required. FlareNet will give a better result.

Based on the back pressure, the PSV type can be selected. For built up back pressure less than 10% of PSV’s set pressure, a conventional type can be used. Balance below type can be used for back pressure up to 30%, and pilot type shall be used for higher back pressure.

Please note, even though back pressure is very important, it is not the only consideration in selection of PSV type. For instance, when reseat pressure of conventional PSV is lower than the operating pressure, it can’t be used even though the back pressure is lower than 10% of PSV’s set pressure. Pilot type is required for this case.

Refer to API 520 part I for more detail about back pressure. Hopefully, this article, at least help you get better understanding of the back pressure and realize the importance of it. I think, we still need more discussion how back pressure has impact to PSV performance, how to simulate it etc. Other time, I will make it as dedicated posting for you.

My friends, I feel that’s all I can share to you for this week. Hopefully, someday we will be a good process engineer. Even though we don’t have many experience recently, we still have the most important; it’s a spirit to grow better. We should take care of it with our life.

Thanks you.
See you next week.

PSV Calculation - Kd,Kc,Kb factor

2011-02-16T11:13:00.026+07:00

Hi friends, how are you today? Hopefully, you are not already bored with my articles. Now, we will learn the concept of relief valve calculation. The spreadsheets not tell you anything, but we should know what really it does. After read this fully article, I hope you get a better understanding of the relief valve calculation concept, and then know well with your spreadsheet.

Pressure is equal to the force divided by the area (P~F/A). That is a very basic concept. I am sure you already familiar with this formula in the very beginning of junior high school. In talking PSV, the force is generated by mass relieving rate. I am sure, all of us know well that relieving rate is depends on cases that to be considered (e.g. fire, blocked discharge, gas blow by etc.)

We need some corrections since the fluid flows through a relief valve nozzle orifice rather than an ideal nozzle. For the same area, at certain condition, the flow capacity of relief valve orifice must be less compared with the ideal one – Its mean that more area is required to handle the same mass relieving rate -. That’s why, there are some correction factors are required such as; Kc, Kd, Kb, Kv, Kp, Kh, Kn etc.

Let’s focus to Kc, Kd and Kb. I will make other posting for the explanation of the other factors.

Effective Discharge – Kd

Kd is effective discharge coefficient used for the mass flux capacity correction for the real nozzle. The higher the Kd value, the closer the mechanical to an ideal design - ideal nozzle, Kd = 1-. It is very obvious that the PSV has Kd value lower than 1. For instance, API relief valve has Kd = 0.975 and 0.65 for gas and liquid respectively.

Kd is depends on the mechanical design. In other word, every vendor has a specific value of Kd. It is important to be realized, so you are not always input 0.975 in the spreadsheet whereas you use PSV other than API.

We can use API data as preliminary calculation only when we don’t have any reference of Kd value. We usually do this at proposal stage.

Some vendors have a better design PSV than standard API. In other word, they have Kd value higher than 0.975. So, I suggest you to ask your vendor the exact value of Kd, especially at project stage. There is potential that the smaller orifice area is required.

Combination with Rupture Disk – Kc

Kc is correction factor when rupture disk to be installed upstream of the PSV.

Rupture disk is required to be installed at upstream of the PSV for system contain solid that may plug the PSV over time. At HAZID/HAZOP, for toxic service, potential leaking of relief valve shall be considered, and then rupture disk at upstream PSV can be used as positive seal for the safeguard. That is why, based on my experience, the combination rupture disk and PSV is very seldom to be applied in the gas processing.

Actually, the Kc value is complex. But I am sure, we seldom use it. So, I am not too interest makes longer explanation. I understand many engineers hate with the long article with something not practical, me too. If you want to know more detail about Kc, please read API 520 part I by yourself.

Impact of Back Pressure – Kb

Back pressure is defined as a pressure existing at PSV’s outlet. It impact to opening pressure, reduction capacity, instability or may combination of all. Kb is required for correction of reducing capacity.

For the low back pressure system that the impact is not significant, conventional type can be used. And for the excessive back pressure service, pilot type PSV is required to overcome it. Then, imagine that low back pressure for the conventional type and no impact back pressure on pilot type due to mechanical design. That’s all, I think, very clear why Kb is required for balance below type only.

Note that the back pressure correction factor Kb for conventional and pilot type is not required, and then use Kb=1. Ask your vendor of Kb value –as graphic- for using below PSV type or refer to API 520 part 1 for preliminary design.

Actually, for conventional type, when the condition is non-critical due to superimposed back pressure, Kb is required. But for now, rather than it will make you confuse, forget it since there are not much system likes that.

The understanding of the back pressure is very important for process engineer. At this time, at least, we know why Kb is required. I will make a separate posting for explanation of back pressure for more detail. I have plan it will have been finished before the end of this month. Don’t miss it.

That’s all I can share this week. The required orifice area calculation procedure is covered in API 520 part I. The standard relief valve orifice is also already stated in API 526 and ASME Sec VIII. Please refer to those documents for the detail.

Thank you.

The Art of Line Sizing

2011-02-10T10:00:00.052+07:00

As process engineer, we have responsibility in determining the pipe size. Both at proposal and project stage, the adequacy of line size shall be reviewed by process team. I am really sure all of us already familiar with line sizing. We know well both the methods and criteria. Here, we will learn together that actually, the understanding of line sizing philosophy is very important. Line sizing is not just about GOOD and NOT GOOD as we can show in the spreadsheet.

Acceptable Size

The acceptable line size is purely depends on the line sizing philosophy. Here, I’ll give some example. The velocity of suction pump shall be keep low enough to prevent vortex at outlet of vessel or tank, and to prevent excessive pressure drop along the suction line. The velocity of reciprocating compressor suction shall be not too high so that leading to the surge. The velocity of drain line shall be maintained high enough to prevent deposit forming. The pressure drop of discharge pump and compressor shall be low enough to prevent high operating cost. The velocity of two phase flow shall not be leading to the pipe erosion. Etc.

Fortunately, many philosophies are already summarized in the parameter that clearly defined. For instance, to avoid deposit at drain line we need velocity more than 1.1 fps, to avoid surge on the reciprocating compressor we need suction velocity less than 40 fps etc. To achieve optimal operating cost of cooling water pump, the water header pressure drops shall be less than about 0.3 bar/100 m. Etc. Those data parameter are very useful. It’s helpful for us to imagine our pipe condition.

It is not surprise for me that those parameters are not the exact value. I mean, the value is not always 1.1 fps, 40 fps. Etc. Here simple example, for liquid sub cooled, suction pump has parameter of maximum velocity 8 fps, we may choose to use 10 pipe size with the velocity 8.1 fps rather than 12 inch with the velocity 7.0 fps. I just say ’may’, not ‘better’ because it is depends on some consideration. Please continue reading. I hope you got it after read this fully article

Many companies make difference in determining the parameter value. for instance, based on my experience, there are various velocity criteria for pump discharge velocity used. Each Client have a specific design criteria. Client A state that velocity shall be less than 29.5 fps, B use 19.6 fps, C say 12 fps, D use 16 fps. Sometimes, the client has his own standard procedure to evaluate economic velocity of the discharge pump. If we refer to API 14E, it stated that economical velocity pump system reached when the discharge pumps velocity is not exceeding three times of the suction velocity. Based on those reality, I think, it’s not forbidden for us to use parameter around those values as long as a good engineering judgment based on experience is used.

Many companies agreed only for two phases flow parameter. They agreed to avoid slug flow and erosion with same parameters called flow induce vibration and erosion. We can refer to the formula stated in API 14E for calculate erosional velocity-[erosional velocity = Constant/mix density^0.5]-. In some cases we cannot avoid slug flow due to erosional velocity limitation. In this case, good understanding of line sizing philosophy is helpful. Actually, slug flow can be acceptable if we can make the pipe stand using more strength support. But, erosion is something we cannot avoid without change the pipe size. Once erosion continued happen, your pipe is under dangerous condition. So, we shall use flow regime and an erosional velocity for limitation of acceptability pipe size for two phase line.

Optimal Size and Get Competitive Cost

At proposal stage, we need to optimize the pipe size to achieve competitive cost.

Regarding of reviewing line sizing result, I suggest for job leader to give more attention for the following line :
1. Large diameter pipe size.
2. Long pipe.
3. Stainless steel or higher material line
4. Line having a large number of valves or and instrumentation.

For example, for large size duplex gas line, carefully reviewing of the line sizing result is very important. Optimize if it is possible. No doubt, the cost will be significantly reduced if we can optimize duplex or stainless steel pipe with many valve installed along that pipe. We can choose the lower pipe size as long as it is reasonable and the philosophy is achieved.

Other example, for air system, there are small pipe size in the system, and the material usually CS or CS galvanized is used. If you have short time of proposal duration, I suggest you to check the adequacy only. Don’t waste your time to optimize this small line.

Sometimes, we are also able to optimize the pipe size for some cases depend on the service condition. Here I give an example.
Look at this picture.

The service line is fuel gas to burner, the line in high pressure, with 8 inch pipe size. If you conduct line sizing calculation, and the result show that NOT GOOD due to pressure drop. We can ignore it. Actually we don’t need the requirement pressure drop limitation. The downstream pressure will be reduced by PCV even more than pressure drop in the pipe. Your spreadsheet always says “I don’t care; my result formula is NOT GOOD”. You must know this. Don’t forget to check velocity or Mach number if you want to reduce this line size.

That’s all I can share to you, let’s keep in our mind, line sizing is not just inputting the data, and then looking at column GOOD and NOT GOOD. Even API 14E says “Calculated line sizes may need to be adjusted in accordance with good engineering judgment”

Hopefully, this article is useful for us. Please correct me if I am wrong. Thank you.

LINE LIST – Input data temperature carefully!

2011-02-07T14:03:00.025+07:00

All of us already know that line list is one of deliverable documents provided by process engineer. It is our important product document as a process engineer at IKPT. Many important data will be covered in this document.

The information to be provided in the line list depends on CLIENT's requirement. Based on my experience, on DEGC Project, ConocoPhillips asked IKPT to put so much information in this document –line name, from-to, operating and design condition, PWHT, NDT, flushing, blowing, and indication of critical pipe etc-. So many of them, right? ConocoPhillips asked all of them since it was project base. On the other hand, when we prepare line list for proposal stage, the whole data does not have to be complete.

Here, I assume that all of us have experience in developing line list -It feel just peculiar when process engineers at IKPT have no experience in developing this document..ha ha ha.just kidding guys-. So, I assume that the explanation about what line list is, how to determine the correct data, etc. are not required anymore.

The data in line list will be used by piping engineer to conduct pipe stress and flexibility analysis (temperature, pressure and density). Pipe stress and flexibility is influenced more by temperature rather than the pressure. Temperature will affect the expansion of both the fluid in the pipe and the pipe material itself. That is why I wrote “Input data temperature carefully “. I mean, we must be sure we have given the correct temperature data.
If you want to know more about what the stress pipe is, you can read here and here

I am not too worried about pressure since the pipe has a specific rating that's generally higher than required design pressure. For example, for pipe which has operating pressure 20 psig, we input design pressure in line list say 22 psig- 10% higher than the operating pressure-. This pipe has rating 150 # which is capable of handling the pressure even more than 150 psig - .

I just say not too worried, it does not mean the correct data is not important. Design pressure in line list will be used as basis of test pressure requirement - 1.5 x DP (internal design gauge pressure) for hydrotest and 1.1 x DP for pneumatic test, for safety reason maximum pneumatic test is 100 psig (or depends on project requirement). You can read ASME B 31.3 for more detail about test pressure -.

Hopefully the following pictures will tell you more about the important of correct data in line list.

Piping engineer has responsibility to provide adequate support and to provide sufficient flexible design. Process engineer, for that sake, has responsibility to provide correct data for that design, then we provide line list.
So, Don’t under estimate that job!

Dry and Wet Gas

2011-02-02T15:20:00.015+07:00

Wednesday, 11 January 2011, DEGC project team conducts regularly meeting. When the issue of PWHT -Post Weld Heat Treatment- requirement for some pipe is raised up on that meeting, piping engineer ask me what the service for and whether the condition is wet or dry.

I said that the pipes are for fuel gas and dry. Actually, I forgot why that pipes are considered as dry gas. But I am sure, they must be in dry condition since the pipes leaving from glycol dehydration contactor.

The consideration of the gas condition –wet or dry- is based on the operating temperature corresponding to its dew point. Some reference state when operating temperature 10F higher than the dew point temperature, it is considered as dry gas. I think, we have an own adjustment with the margin or temperature difference.

Checking the pipe condition by HYSYS, the dew point is 30 F while the operating temperature is 117F, then obviously the pipes are dry service.

Dew point is a condition when the first liquid is being formed from the gas. Dew point temperature can be determined by input vapor fraction equal 1.0 on HYSYS simulator - imagine that the vapor fraction start reduce to 0.999999999, then input 1.0 -. at certain pressure, the dew point temperature will be automatically calculated.

Regarding the dew point temperature, the following service lines require dry condition. That is why, they shall be checked carefully.
1. Pipeline: the operating temperature shall high enough to prevent condensation due to pressure drop along the pipeline.
2. Fuel gas supply: generally, fuel gas supply requires temperature of 20F -30F higher than the dew point temperature.
3. Inlet gas to membrane: liquid to membrane cause membrane damage.
4. Etc. – please comment, if you have other opinion-

In some cases, a heater shall be installed to reach accepted operating temperature to avoid gas condensation.

Preface

2011-01-31T18:20:00.018+07:00

Process engineer should have good understanding of process operations. They must understand how to design the process, how to develop the system and also capable in designing involved equipments in compliance both with the codes and standards. The capabilities in those wide areas may be obtained from self learning and certainly from experience. I am sure all of us agreed that the experience is better.

Here, we can share our experiences that we got at IKPT. Hopefully, we can learn from each others. Although process engineer develops many industrial processes, but in this blog, we will discuss and share something related to oil and gas processing only.

Here, I want to share my knowledge based on my experiences in performing FEED, developing proposals or executing the project. Hopefully, they are useful for us.

Inputs, Corrections, comments and so on in this blog will be warmly appreciated.

This blog is dedicated to my friends, process engineers at Inti Karya Persada Tehnik, IKPT.
Thank to my seniors for shared knowledge and to my little brothers who give me a lots of spirit.
"Friends are the most important ingredient in this recipe of life" its feel true.