Lesson 8

Problems and sizing

After every calculation EasyCAS checks the scheme against good practice and lists what it finds. This lesson spoils the network of lesson 7 on purpose — a main too thin, a consumer with too much pressure — and walks from the counter in the status bar to the pipe size that fixes it.

Three levels, one colour code

Red is red everywhere — in the panel, on the sheet, in the Item tab. A problem that follows from another one on the same item is not listed twice: a pipe too narrow gets one error for its diameter, not a second one for the velocity it causes.

1. Make the main too thin

   Open lesson7.gf9 and save it as lesson8.gf9. Set Diameter of P1 to 16 mm and calculate.

   Row	25 mm	16 mm	Unit
   v, P1	7.87	19.22	m/s
   delta P, P1	≈ 0.10	0.884	bar
   Pout, COMP	≈ 6.17	6.9467	bar

   The main now carries the air of both branches through the bore of a branch. The velocity is more than twice its 9 m/s limit, and the pressure the pipe eats has grown more than eight times — loss grows with the square of the velocity, and the narrow bore adds to it on its own.

   Look at the right end of the status bar: the indicator counts the problems, each level in its colour — Errors: 1  Warnings: 2  Recommendations: 3. The error is P1. The warnings sit on the two open pipes, which now lose more than the 0.5 bar limit on their way from the compressor: Pressure losses from the source to this consumer are above the limit (limit 0.5 Bar; largest loss in ID13 - 0.884 Bar). Between 0.3 and 0.5 bar the same check gives a milder warning, … are high. The recommendations are a catalog pipe for P1 and blow guns for B1 and B2 (lesson 3).

   P1 carries a red badge on the sheet and a dark red row Pipe diameter is at least: the bore at which this air would run at the 9 m/s limit, 23.3846 mm. Below it the hint reads Pipe diameter is below the required minimum.

2. Open the Problems panel

   Click the indicator. A counter opens the panel with that level only; anywhere else on the indicator — the whole list.

   Column	Holds
   Severity	Error, Warning or Recommendation, in its colour.
   Element	The ID, then the label of the card.
   Problem	What is wrong. Checks add the limit and the worst item in brackets, e.g. (limit 0.5 Bar; largest loss in P1 - 0.88 Bar).
   Value	The figure concerned, in your display units.

   Below the list the details pane shows the selected entry in full, with a line What to do. For a pipe too narrow it names the minimum bore; for pressure losses it names the item that loses the most, because that is where a change pays.

   The buttons under the list: Apply (grey here — it applies an audit measure, lesson 12), Size pipe…, Profile…, Go to, Thresholds…, Save…, Send by e-mail… and Close.

   • The coloured counters at the bottom left filter the list by level, like those in the status bar; the active one is underlined.
   • Click a row — the item is selected on the sheet.
   • Double-click it, or Go to — the panel closes, the Item tab opens on the row of the problem.
   • Click a column header to sort; elements sort naturally, ID2 before ID10.
   • When forty items share one problem, they come as one row [+] 40 × …; double-click it to open the group.

   The list belongs to the last calculation. Change the scheme and the panel says the results are out of date and shows only what is still valid.

3. See where the pressure goes

   Keep the row of P1 selected and press Profile…. For a pipe the window opens on the worst consumer whose path runs through it — here B1, marked (worst) in the Consumer list at the top. The graph draws the gauge pressure along the path from the compressor to the consumer: a pipe is a slope, a fitting with a loss but no length a vertical step.

   Line	Means
   blue	The network.
   thick red	The element with the largest loss, labelled with its ID, its drop and its share of all network losses — here P1, 0.884 bar, 93 %.
   grey dashed	A deliberate reduction (a regulator) — not counted as a loss.
   orange / red dashed	Compressor pressure minus the advisory and the limit network loss (0.3 / 0.5 bar by default).
   background	The loss from the compressor: green below half of the advisory loss, yellow up to it, red up to the limit, dark red beyond — the colours of View → Pipe display → Pressure drop.

   The line under the graph sums it up: the number of elements, the length of the path (55000 mm here: 50 m of P1 and 5 m of P2, in the length unit of the Units tab), the network losses against their limit and the element that loses the most. Point at a segment and the second line describes its element.

   The window does not block the program. Leave it open, change P1, recalculate — the graph follows. View → Pressure profile… opens it without the panel, for the worst consumer.

4. Let the program size the pipe

   Back in the Problems panel select the row of P1 and press Size pipe… — or double-click the row Pipe diameter is at least in the Item tab of P1, or use Item → Pipe sizing…. The window lists the minimum bore the calculation found and the candidate pipes from the catalog, each with its pressure drop and velocity, and says whether the path to the worst consumer then fits the network limit. Replace with the selected pipe swaps P1 for that card and recalculates; the position and the connections stay.

   P1 is a generic pipe, yet the catalog still has replacements for it: its rows Count of possible substitutions, Source pressure saving and Annual saving name the recommended catalog pipe — a 1.5 in AIRpipe aluminium pipe in the standard catalog — and what it would save. Size pipe… lists the same pipes. If you would rather keep the generic pipe, just type the bore into its Diameter row — the next step shows how to choose it.

5. Try every size at once: variant calculation

   Right-click P1 and choose Variant calculation… (also in the Scenario menu). In Parameter pick Diameter (mm), set from 16 to 32, points 5, and press Calculate variants. Each variant is a full calculation; Esc stops the run and keeps what is done.

   Diameter, mm	Pout, bar	v in P1, m/s
   16 (current)	6.947	19.2
   20	6.367	12.3
   24	6.188	8.5
   28	6.122	6.3
   32	6.094	4.8

   The table holds the same figures as a comparison of scenarios — source pressure, demand at maximum load, cost of the air, CO₂, consumers short of pressure, errors and warnings — with the best value of each column in green and a chart of the column you choose. The present value is always one of the rows, marked (current).

   Read the curve, not the green cell. From 16 to 24 mm the pressure falls by three quarters of a bar; from 24 to 32 by less than a tenth. The first size under the velocity limit — 24 mm, just above the 23.4 mm of step 1 — is the one to buy; anything thicker costs more metal than it saves air. The last column agrees: 16 and 20 mm still have three errors and warnings, from 24 mm on there are none — while the green cells of pressure and cost go to 32 mm, the thickest one tried.

   Set Diameter of P1 back to 25 and calculate. Scenario from the selected variant… would instead make a copy of the scheme with that value — lesson 12.

6. Give a consumer too much pressure

   Set Pin of B2 to 4 and calculate. In design mode B2 is calculated at 4 bar — about 421 NL/min — but the tee still gets over 6 bar for B1, so B2 really receives two bars more than it needs. The tee T1 notices it: it compares what its branches need, and gets the red row Big pressure difference — a little over 2 bar here — with the hint Reducer required. The row appears when the gap is larger than Allowable pressure drop of the Optimization tab, 0.3 bar.

   Without one, the design figure is a promise the plant will not keep: an open pipe at 6 bar blows 589.64 NL/min, not 421. Insert a regulator — Regulators → MD1-R00-1/8, a Camozzi 1/8 in regulator that needs at least 1 bar across it (dPmin) — between P3 and B2 (release the connection, place it, join it), set its Pout to 4 and calculate: the alarm on T1 is gone, B2 is fed at 4 bar for real, and the design describes what will actually happen. The regulator itself needs 4 + 1 = 5 bar at its inlet (its dPmin), a bar less than the B1 branch, but a branch with a regulator in it is no longer compared by the tee: taking the surplus away is exactly the regulator's job. Lesson 10 measures what happens without the regulator.

   Set Pin of B2 back to 6, delete the regulator and rejoin P3 to B2.

7. Move the thresholds to your own practice

   Thresholds… in the Problems panel holds the limits of the checks. They are stored in the scheme, so each customer's file keeps its own.

   Threshold	Default
   Air velocity — main line / branch	9 / 15 m/s
   Network pressure drop — warning / advisory	0.5 / 0.3 bar
   Minimum ambient temperature	+5 °C (278.15 K)
   Dew point margin	3 K
   Leakage share — warning / advisory (leak orifices drawn on the sheet; the leaks of an audit leak test give a measure instead, lesson 10)	10 / 5 %

   A change acts at once — on the list, the badges and the pipe colours — without recalculating, since the checks work on figures already calculated.

   Check

   With P1 back at 25 mm, set the main line velocity to 7 m/s. P1 (7.9 m/s) gets a warning and turns orange in Pipe display → Velocity. Set it back to 9: both go.

8. Read the recommendations

   A green ↔ row means a better catalog component exists; the item carries a wrench. Double-click the row: the list of replacements opens with the recommended one first (lesson 3). The first candidate is checked against the whole network during the calculation, the others when the list opens or the report is built — until then the row Substitutions not yet checked counts them. Esc during a calculation skips that check; the results stay correct.

   The Optimization tab sets how picky the search is: Allowable pressure drop of a branch, Tolerance for replacement in per cent, and Show marks on the scheme for all badges. View → Show warnings on the scheme hides only the orange triangles.

9. Hand the list on

   Save… in the Problems panel writes the whole list, regardless of filter and sorting, as a PDF or an Excel workbook — choose the type in the file dialog. The workbook has a column per field, so values can be sorted and summed. Send by e-mail… prepares a message with the PDF attached. The PDF report (lesson 16) carries the same list near its top.

   Save lesson8.gf9 with P1 at 25 mm.

When it does not work

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