Pump Curve Question 4

[rhatcher]

Can anyone tell me what the triangles on the attached pump curve represent? I am thinking that they represent the 'design requirements' for the application that the pump is being tested for. The application in this case is a dry dock located on a tidal river.

 

[waterpipe]

The link doesn't work. But I can guess what triangles you mean. they are right on the curve?
They show the design point. The point that the pump is working in a specific project. normally this is drawn by mfg in its proposal.
please update the link so we can make sure about this.

 

[rhatcher]

Thanks for the quick reply waterpipe. I've tried to attach the file again.

It sounds like you are confirming what I thought (ie. triangle = design point/design requirement). I was thinking this is the case because one of the triangles corresponds with the pump rating given on the pump curve sheet even though the rating and the triangle do not fall on the curve (??).

Please look at the curve and comment.

 

[rhatcher]

Another try....

 

[rhatcher]

My 13:42 post has a working link to the pump curve. For everyone's future reference, the original file name was 'DD#1 Pump Curve' and apparently the uploading feature did not like the '#' character in the name.

 

[waterpipe]

O.k, This could be the real readings from a hydraulic performance test shown on the curve, especially as there are certified, approved and witness signatures.

 

[rhatcher]

I attached the rest of the data that I have. It would appear that the pump was tested at 118rpm and that these data points were used to calculate the pump curve at 260rpm. The little crosses on the curve represent the various test points.

I am thinking that the pump rating of 47000 gpm and 40 feet of head is a design point for the dry dock application since it does not correspond to any of the test points. The 40ft head may be the high tide point and the 35ft head may be the low tide point(?). This is what I am trying to determine. Are these (possibly) the design points for the dry dock application?

 

[Artisi]

I wouldn't think anyone here is privy to the design info' of the dry dock pump spec however, can we assume that the pump duty is to empty the dry dock.

As for the 2 duty points, it is unclear unless you give more info'.
Furthermore, as this is a test curve it appears that the pump has over performed against the required duty, why? - again it is unclear but at a guess I would say that it is a standard build/configuration and this is its standard performance.

To answer your original question, yes, the triangles are the normal way of marking "duty point/s".

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[rhatcher]

Thanks to artisi and waterpipe for answering my question about the symbology of the triangle.

As artisi suggested, there are a lot of questions about this application and that is the reason I am looking so hard at the pump curve.

 

[Artisi]

You need to be looking at the application, the pump is the pump and it will perform to suit the installation, that is assuming it has been correctly supplied for this particular application.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[rhatcher]

Artisi,

With regard to the application, the pumps (two identical units) do successfully remove water from the drydock. The problem is that the motors draw overcurrent and run hot. The application and the associated problem have obviously existed for quite some time.

Upon examination of the pump curve and the motor specifications, it appears that there is a mismatch. Specifically, the pump rating corresponds to 700hp at 260rpm and the motor is rated at 700hp at 267rpm. To further compound the mismatch, when I had the motor checked with a tachometer the actual running speed is 270rpm.

I do not have my calculations in front of me but...at the 21.23 ft head level (maximum motor power on the data sheet) I estimated that the required pump power for the increased speed is about 784hp or a 12% overload. In addition, since the motor power curve is relatively flat, the overload exists to some degree throughout the pump cuve. For power calculations, note that the application is salt water = 1.025 specific gravity.

During the process of figuring this information out I found myself continually looking at the triangles and wondering what they represent, especially since the one at the 47,000gpm point corresponds to the pump rating but does not fall on the pump capability curve.

I subsequently came to suspect that the triangles may represent the design requirements for the drydock application.

I agree that the pump perfomance will be based on the application design requirements and not the pump capability curve, especially if the application design points do not fall on the pump capability curve.

This is the reason that I want to know what the triangle points reresent. If they represent the application design points, then this is, in my opinion, further evidence of a total application/pump/motor mismatch.

What do you think?

 

[Artisi]

I have only just noted the 3rd duty point on the extreme right of curve, and can only now assume the 3 points represent the anticipated flows and heads with the dock at maximum and minimum water levels.

According to the curve the calculated power at 260 rpm is max 700hp, if in fact the installed motor/s are 270rpm then power required will exceed 780hp, maybe even get to 800hp as the speed increase will also develop more head with a further increase in flow rate.

Guess you have 2 options
1. impose more head against the pump using a gate valve or orifice plate
2. reduce the impeller diameter to suit the higher speed

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[rhatcher]

Artisi,

Thanks for continuing this discussion.

I am thinking that the third design point at about 21 ft head is the reason that the pump seems over rated at the design points for 40ft and 35ft head.

Specifically, in order to fit the pump capability curve to the design points as represented by the triangles, the third 20ft head design point has to be on the curve or under it. The shape of the curve then leaves the first and second design points, 40ft and 35ft head, well under the pump capability curve.

The dock is rated to draft vessels up to 30' in keel depth. The dock draft requirement includes allowing for the vessel to float over the supporting blocks that are, on average, 5-10' high at the keel. This makes the first two design points, 40' and 35' head, seem reasonable as representing the difference between high tide and low tide for an almost empty dock.

At this point I will add that my recollection is that the piping diameter is larger than the pump suction/discharge diameter by up to 20" (150%) and that there are two 90 degree bends between the pump discharge and the piping discharge to the river. This makes it seems reasonable to assume that the dynamic friction head loss is relatively low.

This leads to a question that I have been pondering. When the dock is full, the static pump head is essentially zero because the level of the water inside the dock is equal to that of the river outside of the dock.

This makes the third design point at 21ft head seem unreasonable as representing a full dock with zero static head since this implies a dynamic friction head loss of 21ft of pressure. If the friction loss was actually 21ft of head pressure then the 40ft deep dock would only be able to pump about one-half empty.

It would seem to me that the actual operating point for this pump with a full dock, assuming zero static head pressure and minimum friction loss, would be beyond the pump capability curve as shown. If I extrapolate the pump capability curve out to near zero static head and allow for the dropping efficiency then I estimate the flow would be in the range of 85000gpm. This estimate is based on the original curve and does not include the motor speed difference. I haven't performed specific calculations for this data point yet.

Does my analysis of what is going on make sense or do you see something different?

As a point of interest, I have included an aerial photo of the dry dock in question. It is located in the bottom center of the photo with a tanker ship docked that has a black hull, burgundy deck, and white superstructure. The dry dock dimensions allow for a vesssel that is up to 622ft long, 97ft wide, and with a draft of up to 30ft.

 

[rmw]

It is a floating dock, right? Where are the pumps, on the dock or besides the dock on a stationary pier or the like?

rmw

 

[rhatcher]

rmw, the dry dock is a permanent structure with the bottom of the dock approximately 35-40ft below the level of the river (sea level). A floating caisson (lock) is used to seal the dry dock before it is drained. The pumps are located underground adjacent to the dock. See the picture as described at the end of my 21:34 post.

 

[Artisi]

How about a sketch of the installation.

Can you get hold of the original design data/ pump spec etc so we can make some sense of what is going on.

Irrespective of the above and the water levels etc, if the pump is running at 270rpm you can expect overload.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Pumpsonly]

rhatcher,

The 3 triangles are normally duty points specified by the purchaser as the worst conditions.As a dry dock pump will operates with increasing static head as soon as it is started, the motor sizing may have been selected to be overloading at the beginning of the run to save capital cost.
However, the friction losses of 21 feet head when the dock is full may have been over estimated as in most pump purchase specification.
Assumed you are operating at 60Hz power supply with a 24 pole motor, the 260 RPM rated speed as stated in the pump curve is also seems too low to me and thus compounding your problem of motor overloading.
How much is the motor over current and for how long? what are the information given in the motor nameplate?
From the pump test date, it seems that the pumps / motor have been working almost 20 years now. Has the motor ever burn out?

 

[DubMac]

Before thinking about the application, lets just look at the points on the curve:
The triangles are duty points that were calculated by design engineers before any pump was ever selected. These points went out to bidders and obviously the pump described on the curve was selected and purchased back in '89. Factors other than best hydraulic fit certainly could be in play, i.e. price, availability, preferred vendor, etc.
The curve shown was generated from data points taken from performance test in the factory on the actual pump purchased. Test was run at 118rpm and the data was converted to 260rpm using pump affinity laws. I think I saw that the pump tested had a 48" impeller diameter (on data sheet). Before the pump was shipped out, it very well could have had this impeller trimmed to somewhat smaller diameter to match the design point more closely. Typically, pumps will be tested at full impeller diameter so there will be reserve diameter in case pump underperforms; then trimmed afterwards to match duty points. The actual diameter of the impeller, as shipped, should be on the nameplate of the pump and this diameter will tell you how the pump should actually perform. You should definitely also get an original “generic” pump curve for that pump from the manufacturer to see the range of impeller diameters and horsepower requirements of the pump.
In terms of overamping the motor, 9 times out of 10, this is due to allowing the pump to run too far out on it’s curve (x axis) to a point where both the HP requirements are too great for the motor and/or the NPSHA falls below NPSHR at that capacity. If there is some type of valving in place that can regulate the discharge flow so as not to let it exceed say 60,000 or so GPM, then the motor should never overamp.
It also must be considered that the pump has been modified over the years or the impeller or internals may be worn to such a point that the original capacity curve is meaningless. I would definitely suggest doing a field performance test; they are fairly easy if you have access and some time to play with the pump.
Gotta go, hope this helps some.

 

[chicopee]

first (from left)triangle if the 100% rated point, the middle triangle is the overload (@150% of rated load) point and the third(right most) triangle is I believe called the break point.

 

[Pumpsonly]

rhatcher,

Just realized that you are have a 26 pole motor with a full load speed of 267 RPM. If you measured a 270 RPM at site, means is not over loaded, unless the power supply frequency is higher than 60Hz.What was the current drawn at the time of your measurement and compared with the rated motor FL amp? As I mentioned earlier, the pump / motor will draw the highest load at the start of the run and reduces as the water level in the dock decreases as the flow rate reduces with increasing static head.