pipingdesigners.com Training Seminar

Section - II
C-II: Vertical Vessel Orientation
By: James O. Pennock

The following article was prompted by questions from a young piping designer. He wrote:
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Hi
I am getting ready to do my first vessel nozzle orientation. The vessel is a Stripper Tower (a). Can you help me? First, what are the things I have to take into consideration? Second, what are the key steps in the process for doing a vessel nozzle orientation?
Regards
XXXXXXXXXXXX
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(a)The name/function of the vessel has been changed.

For your first question: "What are the things I have to take into consideration?"

The answer to this question is very simple; you must take everything in to consideration. Everything is important! Someone may tell you that some things do not matter but this is not true, everything matters.

You need to consider the following:

a) Timing: Vessel orientation is normally the only equipment related layout activity that can be done without specific input from a vendor. All of the information required for vessel orientation is generated on the project in the form of P&ID data and project standards. It is also one of the few activities that will feed one or more other downstream groups whose work is critical to the project schedule. With this in mind this activity can and should be started as soon as the P&ID reaches "Approved-For-Design" (AFD) status. Te vessel orientation activity can be started manually or on basic 2D CAD before the 3D PDS data base is fully loaded and checked. There is some logic to doing this activity manually or in 2D CAD because of the amount of trial and error required to finally achieve an acceptable and approved orientation. Once the orientation is approved and the PDS data base is ready the 3D model can be built with no recycle.

b) The Plot Plan (Note 1): The plot plan is required to identify the location of the vessel and its related equipment. The related equipment includes the equipment that feeds the vessel (is up-stream) and also the equipment that the vessel feeds (is down-stream). It shows and locates adjacent, non-related equipment. It also shows adjacent structures that may support the related up-stream or down-stream equipment. It also indicates the plant features such as pipe racks, operating aisles, maintenance access areas and the direction of Plant North.

c) The project foundation criteria: Vertical vessels normally sit on an octagon pad foundation with the top of grout at EL101' - 0" (high point of finished paving = EL 100' - 0"). You need to have and understand the type and elevation of the foundation for this vessel.

d) The P&ID's (Note 1): The P&ID's are required to show the process streams that connect to the Stripper Tower and its related equipment. In my experience P&ID's are much like the pages in a book. Some equipment (the heater) starts or shows on sheet one P&ID the story continues with the key item (the Stripper Tower, Thermosyphon Reboiler and Bottoms Pumps) showing on sheet two and then continues to some conclusion (the overhead condensers) on sheet three. You will need all three process system P&ID's. The Stripper Tower P&ID will show a graphic of the column along with all the piping connecting to the vessel. There will also be a data block at the top of the page. This data block should include the vessel number, the vessel name and the basic size. It will also indicate the design temperature and the insulation requirements (if any). The graphic of the vessel should also indicate the basic type of internals (Trays or Packing). If the internals are Trays then the number of trays should be indicated. The trays just above or just below where a line is connected should be numbered. If the internals are some form of packing then the extent of the packing beds should be indicated.

e) The project Line List (Note 1): The line List is required to give you specific and critical key data about the lines such as the Line Number, line class, maximum operating temperature and insulation requirements,

f) The project Piping Material Specifications (Note 1): The Piping Material Specifications are required to give you the data about metallurgy and any specifics about fittings, flanges, valves or requirements for PWHT (post weld heat treatment).

g) The Vessel Drawing (Note 2): The vessel drawing at this time will most likely be marked "Preliminary." It will give you; the inside diameter (I.D.), the tangent-to-tangent shell length, the shape of the top and bottom heads and the skirt height. This drawing should also have a table showing all the nozzles with the basic information such as: identification, quantity, and size, flange rating, the elevation above (or below) the bottom tangent line for each nozzle, the purpose for the nozzle and any special instructions. The vessel drawing needs to also indicate where the internals start and end inside the vessel.

h) The Internals (Note 2) (Trays or Packing) A tower can have a number of different types and configurations of internals. It may be Trays or it may be some form of Packing.
- Trays: If you have Trays then you need to know: the number of trays, the spacing of the trays, the number of passes for the trays (1-pass, 2-pass, 3-pass etc.). You also need to know if there are any "draw sumps," baffles or other special features.
- Packing: You need to know the number of "Beds," the depth of the beds and the method of installing and removing the packing material. You also need to know and understand about the type of feed distributor(s) to be used. You need to know about the packing discharge nozzles.
For the purpose of this article we will assume we have 35 single pass trays.

i) The Thermosiphon Reboiler data sheet (Note 1): This will give you the preliminary size and type information. The P&ID indicates that this vessel has a vertical Thermosiphon reboiler fitted to it. Some discussion should normally take place to determine the optimum tube length and the proper support elevation and support method.

j) The project Vessel Platform Standards (Note 1): This will give you the required information about the minimum vertical spacing between platforms. It will also give you specific details about platform supports and how to make the openings where pipes must pass through a platform. This drawing will (or should) also give you specifics about handrails.

k) The project Vessel Ladder Standards (Note 1): This drawing will give you all the required information about ladder construction and more important the limits for the maximum vertical run for a single ladder.

l) The project Vessel Nozzle Standards (Note 1): This will give you all the normal options for un-reinforced and reinforced nozzles. It may also show you some options for internal nozzle piping.

m) The project Vessel Davit Standards (Note 1): A davit is a small device permanently mounted on the vessel that acts as a crane for lifting heavy objects such as tray sections.

n) The project Vessel Pipe Support and Guide Standards (Note 1): These are devices attached to a vessel that support and/or guide the vertical runs of pipe. This drawing also defines the minimum distance from the outside of a vessel shell to the back of an adjacent pipe. Where I came from this was called the "L" dimension. The "L" dimension was normally 12" (adjusted as required for insulation) The maximum was 20" without a special design. The key was to have a minimum of 7" clear between two co-existing insulations. These supports and guides also require a wider than normal line spacing in the vertical plane as the lines go up or down a vessel. This is mainly due to the configuration of the Trunnion (Note 3) support attached to the pipe and the pipe clamp used for the guide.

o) The project Piping and Vessel Insulation Specification (Note 1): From this document you will get the thickness of the insulation needed for the pipes and vessel at the operating temperature.

(Note 1): These items are normally created by your company for the project and should be "Approved for Design" (AFD) quality documents. This means that they have been through all of the proper in-house reviews and checks and have then been approved by the Company and the Client for use in the design of the work.

(Note 2): These documents will initially come from the project Vessel Engineer. They will normally be marked "Preliminary" until they receive and process your orientation drawings. Later you may receive the vessel fabricator's detail drawings for "Squad Check" (review and approval).

(Note 3) For more information about a Trunnion support see www.pipingdesigners.com look under Training and Secondary Pipe Supports

There may be other documents that are required due to a specific company's method of operation.

The next things you need to consider is; functionality, safety, operation, maintenance and constructability.

Functionality: No matter what, this vessel must do its job. You must know and understand what that intended job is. You do not need to be a process engineer but you should be involved in the review of the P&ID for this specific vessel. You need to hear what the critical issues are relating to this vessel and the connected piping. If your company does not include piping in the formal review of the P&ID's then you need to seek out the process engineer and ask him or her to explain the function, key points and any critical issues relating to this vessel.

Safety: This is the other important issue relating to vessel orientation. The operation must be able to be done in a safe manner. The same must be said for both maintenance and constructability. To achieve this goal the locations of nozzles relative to the placement and arrangement of the ladders and platforms must be carefully considered. The travel path (access and egress) must be arranged so the main travel path cannot be blocked by open manholes, scaffolding, tools, tray parts, valves or piping. The basic rule here; a: ladder #1 comes up with a side step-off (right or left) on to platform #1. Then b: there is a minimum rest space equal to one ladder width. Then c: the next ladder (#2) continues up to the next platform. Platform #1 can continue beyond ladder #2 around the vessel to provide access to nozzles and manholes. This arrangement does not impede or obstruct the clear path for rapid escape from the vessel for anyone from a higher elevation. Other safety issues include one or more skirt access openings located near grade which should be located with clear access. There will also be four or more skirt vents located high near the skirt-to-vessel attachment which also should not be blocked.

Operation: Process plants need to be operated. Most operation is concentrated around valves and instruments. These items must be accessible. Accessible means reachable. This reachable is conditional. Nozzles with a nominal size of 2' (NPS) and smaller can be reachable from a ladder or from a platform. Nozzles 3" (NPS) and larger shall be reachable on a platform. In this context the from means that the object is not more than 18" (one arms length) from the ladder or platform and the on means the object must be fully inside the platform. There is normally only one exception to this rule. That is for valves or nozzles that are located less than 20 feet from grade and can be accessed with scaffolding or a "Man-Lift".

Maintenance: All the accessibility issues that apply for operations also apply for maintenance. In addition don't block access to manholes with control valve assemblies or other piping. Make sure the Electrical and Instrument people don't locate a panel or a transmitter assembly in the operations or maintenance access ways.

Constructability: This vessel needs to be erected and therefore it will need Lifting Lugs. These are normally very large steel shapes with "eyes" welded to the top head. They will normally not interfere with your orientation, however you should check to make sure.

Your second question: "What are the key steps in the process for doing a column nozzle orientation?"

The key steps in the process are:
(You may choose for some reason to do something in a different order, but this is how I think I would do it. It should be noted that I like to be able to have all things numbered from the bottom up. This includes trays, nozzles, ladders, platforms, etc. However, sometimes due to company preference or the tray manufacturer standards the trays are numbered from the top down.)

1. Data collection - Collect a copy of all the drawings listed above. Make a folder file (or a stick file) to keep them in. Mark all the drawings "Stripper Tower Orientation Master" (STOM). This STOM file is your justification for everything you do or did. If anyone has reason to question why you did what you did then you have a file of the source material you based the work on. It is your responsibility to use the proper information and to properly file and incorporate changes from all new revisions when received.

2. P&ID conditioning - Take your STOM P&ID and pick-up any marks from the Project Master copy. From time to time as you work, go back and recheck the Project Master P&ID for any new marks (i.e.: line size changes, additions, deletions, etc.). Study the Stripper Tower and identify all the related equipment and all connecting lines. Study the lines for valves and instrumentation.

3. Plot Plan conditioning - Take the STOM Plot Plan and with a yellow high-lighter identify the Stripper Tower and all the related equipment. Related equipment means that which is directly connected by pipe to the Stripper Tower. I prefer to work with Plant North up or towards the top of the paper (CAD screen). When I do a vessel orientation I consider the pipeway to be in "front" of the vessel. I call the maintenance area the "back" of the vessel or equipment row. For the purpose of my instruction here I am going to assume that 0º is "up" and "up" is north. Maintenance is on the north (back) side and the pipe way is on the south (front) side.

4. Prepare preliminary elevation - Manually or by CAD, create a scale drawing of the vessel elevation (side view) Locate the bottom tangent line and in phantom (dotted line) the bottom head. Accurately locate the top tangent line from the bottom tangent line and draw in the top head. We will assume that this vessel is a skirt supported vessel and that the skirt is 20 ft high. (If not skirt supported then Leg or Lug supported will require optional considerations that we can discuss if applicable.) At the bottom accurately create the skirt (vessel support). Check with the Structural department and find out how high the foundation is for this vessel. Make sure they give you the top of grout (TOG) not just top of concrete. They are not the same. I will assume that the TOG is EL. 101' - 0." Now indicate the high point of finished paving (HPFP). I will assume that the HPFG is EL. 100' - 0." Now from this HPFP line, draw a light line to indicate the projects minimum head clearance.

5. Prepare preliminary plans - Manually or by CAD, create a scale drawing of a number of plan views. The plan views will be where you will do most of your work so make one for each ten feet +/- (3 to 4 meters) of vertical elevation ending with one above and showing the very top platform. These starter plans should have crossed center lines and the actual I.D. of the vessel. (We are using 8' - 0" for this article). Mark the location of Plant North on each mini-plan. Normally plant north is the same as 0 degrees on the vessel shell. East is 90 degrees, South is 180 degrees and all additional orientation is clockwise from north and 0 degrees. Don't worry about the O.D. or the wall thickness. Now, look at the platform drawing and get the clearance from the vessel shell and the inside edge of a platform. Draw a very light circle (different color and/or layer) on each mini-plan to indicate where the inside edge of a platform might be. Now draw another very light circle 3'-0" (1meter +/_) more in diameter to indicate where the outside of a platform might be. These are not real platforms yet they are just guide lines to remind you of platforms as you do other work. Now mark the "Front" (pipeway side) of the vessel and the "Back" (maintenance side) of the vessel.

6. Thermosiphon Reboiler: The Reboiler for our sample vessel has a 42" shell, 24 ft fixed tube (vertical mount) shell and tube exchanger. The shell side is high temperature steam. The tube side is the process fluid from the bottom of the tower which enters at the bottom end of the reboiler. The process vapor exits the top end of the reboiler and returns to the tower below tray #1. The placement and support of the Thermosiphon Reboiler is the next thing we should cover. Because of the plot plan placement of our Stripper Tower the Thermosiphon Reboiler will be mounted directly to the tower at the 270 degree point. It will have a knee braced cantilevered support that is attached to the vessel. The exchanger needs to be supported so the top tube sheet is at the same level as the high liquid level inside the vessel.

7. Bottoms section baffle - Because of the way this vessel works there is a baffle dividing the bottom section of the tower. The baffle can not be on the centerline of the vessel because the reboiler feed nozzle is centered on the bottom head. Therefore the baffle must be offset to miss that nozzle connection. The height of the baffle is the same as the "High Liquid Level." All of the liquid that comes off the downcomer from tray #1 goes into the "large" side of the bottom section. It then goes through the reboiler and returns to the vessel as vapor. Excess liquid from the "large" side overflows the baffle and becomes the "Bottoms" and is drawn off by the bottoms pumps. The connection for the bottoms nozzle "B" is on the "small" side of the baffle.

8. Check for nozzle continuity - Look at the STOM P&ID and the table of nozzles on the vessel drawing. They should match in number and size. In pencil mark each line connecting to the P&ID vessel with the nozzle number from the vessel nozzle table. Do they match in number? Do they match is size? If not, go see the Process Engineer and ask for clarification.

(Sample) Stripper Tower Nozzle Table

The bottom tangent line elevation = 121' - 0"
The top tangent line elevation = 232' - 8"

9. Check for nozzle temperature - You now have all the nozzles connected or identified to its specific line. Now look at the line list and fine the maximum operating temperature for each of the flowing lines (feed and main outlet lines). Don't worry about vents and drain. In pencil, mark these temperatures onto the STOM P&ID at the point where the line connects to the vessel. You now have the vessel identified, the line from somewhere connecting to the vessel, you have the connection point identified with a nozzle number and you have a temperature at that nozzle.

10. Locate nozzle elevations - Based on the elevation for each nozzle (given in the Nozzle Table on the Vessel Drawing) locate all the nozzles on the scale vertical view (side view) of the vessel. Most of these flowing lines will be above the bottom tangent line. What this means is that all things connected to the nozzles above the bottom tangent line will grow up when the vessel is hot and in full operation. Only four of the nozzles are located below the bottom tangent line and these nozzles (and their attached piping) below the bottom tangent line will grow down when the vessel is hot and in full operation.

11. Establish temperature zones - The next step is to calculate the incremental and total vertical growth of the vessel. The incremental growth means the growth for a specific section of the vessel. Trayed vessels do not have the same operating temperature from bottom to the top. They have a graduated temperature. You may be asking what temperature you use for this operation. DO NOT USE THE VESSEL DESIGN TEMPERATURE. The vessel design temperature may be something like 500 degrees F. If you use this number along with the height of the vessel and the coefficient of expansion for the vessel metallurgy you would end up with a total expansion that would be incorrect. You look at the temperatures you marked for each of the Flowing lines. You take two adjacent Flowing nozzles that have a temperature. Let's say we take the Feed nozzle and the Bottoms Out nozzles. (I am assuming there are no other flowing nozzles between these two nozzles. If there are then make the appropriate adjustment). These two nozzles and their temperatures form a zone. You add their two temperatures together and divide the answer by 2 to get an average temperature for the zone (example: (475 degrees F and 395 degrees F)/2 = 435 degrees F). You use this 435 degrees F figure for the maximum operating temperature along with the zone length and the coefficient for the vessel shell material for the calculation of the incremental expansion. Do the same for each set of flowing nozzles and calculate the incremental expansion for each zone. The overhead vapor line temperature may be as low as 180 degrees F. Somewhere lower down the vessel there is another flowing nozzle with its operating temperature. This forms the top zone in the group. For talking purposes let's say we have five zones. Let's say that Zone one expands a total of 1", Zone two expands ¾". Zone three expands ½", Zone four expands ½" and Zone five expands ¼" for a total of 3". You need to mark each of the incremental expansions at the appropriate place. Now take each of the incremental expansions and add them together as you progress up the vessel. Part of Zone one is below the support point so some of the expansion grows up and some of it grows down. Because of this let's say that the top of Zone one only grows up 5/8" during operation. The top of Zone two grows up a total of 1-3/8". Zone three grows up a total of 1-7/8". The top of Zone four grows up a total of 2-3/8'. And the top of Zone five grows up a total of 2-5/8". You also need to mark each of the accumulated expansions at the appropriate place. You now have a basis for the preliminary pipe flexibility work you will do later.

12. Locate manholes - We have three manholes and they are only used during maintenance. These manholes will be the hinged type and for our situation they will all open to the right. They are identified as M#1 (bottom section) through M#3 (top section). They are not used or needed during operations. So Manholes should normally be located on the "back" side of the vessel. This is logical and it works 90% of the time. One of the times it does not hold true is for the lower shell manhole when there is a vertical Thermosiphon reboiler attached to the back of a vessel. So you can start with all of our Manholes on the back centerline of the vessel. This may not be the final location but it is a starting point. From the bottom of the vessel M#1 is in what is called the "surge" section. There are (normally) no internals in this section. So if we need to we can locate M#1 at any orientation. M#3 is in the very top section above the top tray so it also has few limits to its orientation. Manhole M#2 is located between trays at a maximum spacing of (say) twenty trays. In our case M#2 is on tray #19. The side manholes need to enter on a tray, not behind the downcomer.

13. Steam out nozzles: Along with each manhole there will also be a steam out nozzle. This nozzle will be fitted with a valve which will be blind flanged. During shut-down the blind flange is removed and a flanged spool with a steam coupling will be installed. Prior to any entry into the vessel the steam will be turned on for 12 to 24 hours to remove (steam-out) hydrocarbons. The steam-out nozzle will be located in close proximity to the manhole. The recommended placement for the steam-out connections on our vessel will be to the right and 1' - 6" below the manhole center line.

14. Set tray orientation - As we said above, we have 35 single pass trays. Tray #1 is 35' - 10" above the bottom tangent line of the tower and tray #35 is 104' - 10" above the bottom tangent line. Since we have trays that have only a single pass (downcomer) then we have almost 270 degrees of orientation with which we can place the manholes. However that 270 degrees of orientation needs to be in the right quadrant. If the excluded part of that circle is centered on 0 degrees (North) then we need to ask if that manhole can move up one tray or down one tray. If we have trays that are two pass or three pass then we need to find ways to orient the manholes, nozzles and trays so they co-exist. We have located all our manholes on the maintenance (north) side centerline at 0 degrees. We will then place the orientation of the trays on an East/West center line. We then insure that we adjust the vertical location of the manholes (up or down one tray) to enter on to a tray.

Up to this point you have doing the very important background work that is required before you can do the actually vessel orientation. Next you need to locate the nozzles, determine where the pipes will travel up or down the vessel and establish the support and guide points for each line. As you do that you also need to establish the ladder and platform requirements to provide proper access for operation and maintenance.
So let's move on to the next task.

15. Nozzle placement - As we stated before large nozzles need to be accessible "on" a platform. So keep that in mind as you proceed. Start with the nozzles at the top of the vessel and work down. Here is a key to remember, the line (up-or-down the vessel) and the nozzle do not need to be at the same bearing point. By this I mean that the line up-or-down the vessel can be at one point, say 196 degrees, and then wrap around the vessel to where the nozzle is on the other side of the vessel say at 315 degrees. The line would rise up the vessel and then turn horizontal to go around the vessel. It would then turn vertical again, go through the platform required for nozzle access and then enter the nozzle. This allows the nozzle to be "on" a platform but the line does not penetrate all the other platforms. Nozzles "F" and "R" on this vessel might be done using this method. The other lines from the "V1' nozzle and the PSV can simply drop down the vessel at the most convenient point. The lines to and from the Thermosiphon Reboiler will connect almost fitting to fitting with no valves. The bottoms line to the pumps is also a simple routing and might exit the vessel skirt at the 90 or 180 degree point depending on where the pumps are located. Instrument connections need to be placed so they perform their function and so they are accessible from a ladder or a platform. They do not normally extend far from the vessel shell thus do not cause an obstruction so with care they may be positioned on the vessel in the space between two ladders.

16. Pipe Supports - Each line that travels up or down the vessel will need one or more pipe supports. Lines that travel up-or-down the vessel at the same bearing point as the nozzle only need one pipe support. For side mounted nozzles this support will be located a short distance below the top elbow. For top mounted nozzles the support will be located a short distance below the vessel top weld seam. Lines that travel up-or-down the vessel at a different bearing point as the nozzle need to be considered for two supports. One below the nozzle elbow and a second support below the elbow where the line drops down the vessel.

17. Pipe Guides - Each line that travels up or down the vessel will need to be considered for pipe guides. The two factors in determining the number of guides a line requires is the wind force at the jobsite and the length of vertical travel. Some lines require only one guide and others require more than one pipe guide. Each line that travels up-or-down the vessel normally turns (elbows) horizontal at some lower elevation. The bottom guide should not be placed closer than 50 pipe diameters above this elbow. Other guides for a line may be spaced by taking the elevation of the support (at the top of the line drop) and then deduct the elevation of the bottom guide. The space remaining is then considered for one or more additional guides. Guides should be spaced every 20 to 30 feet.

18. Ladder placement - All of the ladders should be placed in the same general quadrant of the vessel. It is simple to work out the minimum spacing from one ladder to another. As stated before the minimum space between two ladders should be equal to one ladder (measured at the center of the cage). So if the ladder (with cage) is 2'-6" +/- wide then the space between two ladders is also 2'-6"+/-. This makes the center to center between two ladders 5'-0"+/-. Most of the ladders on this vessel can be in the quadrant from 45 degrees to 135 degrees. For a vessel 8' - 0" in diameter this would mean:
- Ladder #1 would be at 135 degrees
- Ladder #2 would be at 90 degrees
- Ladder # 3 would be at 45 degrees.
- Ladder #4 is back at 135 degrees.
- Ladder #5 is at 90 degrees and
- Ladder #6 is at 45 degrees.
- There will be a ladder #7 on this vessel which we will discuss when we talk about platforms.

19. Platforms - Platforms are the next thing to be defined.

Platform #1 would start at the step-off from ladder #1 (135 degrees) and wrap around the vessel (counter clock wise) to about the 350 degree point, beyond Manhole #1.
Platform #2 would start at the step-off from ladder #2 (90 degrees) and wrap around the vessel (counter clock wise) to ladder # 7 located at 315 degrees. Ladder #7 goes both up and down to provide access to two auxiliary platforms #2a and #2b. These small maintenance platforms provide access to the head flange of the reboiler and to nozzle N2. They must be sized to meet the criteria that the nozzle and head flange is "on" the platform.
Platform #3 would start at the step-off from ladder #3 and wrap around the vessel (clock wise) to and under ladder #4 at 135 degrees.
Platform #4 would start at the step-off from ladder #4 and wrap around the vessel (counter clock wise) to about the 315 degree point for access to Manhole #2 and to provide maintenance access for nozzle "F".
Platform #5 would start at the step-off from ladder #5 and provide a minimum platform (counter clock wise) for access to ladder #6
Platform #6 would start with a side step-off from mid way up ladder # 6 and wrap around the vessel (counter clock wise) to about the 315 degree point for access to Manhole #3 and to provide maintenance access to nozzle "R".
Platform #7 is a "Top" platform supported from the vessel head. This platform must be sized to allow space for the piping off the vessel head, access to the Davit and room for maintenance people to work during turn-around.

The imaginary vessel we have been discussing above is really a very simple vessel. After you read all of this you may think that vertical vessel orientation is very complex. You are right! However, I think vessel orientation is also the most fun there is in all of piping design.
For those of you who may want to try this vessel as a trial run I say give it a shot. Please feel free to E-mail me at (jopennock@netscape.net) when you start and maybe I can offer some suggestions.
Good luck to all of you who get a chance to do an actual vertical vessel orientation.

James O. Pennock is a former Piper with more than 45 years experience covering process plant engineering, design, training, pipe fabrication and construction. He is now retired and lives in Florida, USA.