Valve Actuated Water Heater Example

Hello, last question of the night, I was given a problem that is basically teaching me how to use comparator operators...

I've got a general idea of how to do this which I'll include at the bottom, but I was hoping for some input or corrections to what would work and what wouldn't.


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Problem: I need to work out the ladder code for a valve actuated water heater on a SLC-500 with the following data...

When the machine first turns on, the pilot valve (PV_ opens and the igniter (I) will turn on for 3 seconds.

A thermal switch T1, indicates whether the water is hot or not. If the temperature of the water is less than 95 degrees C, the switch is closed.

A second thermal switch (t2) opens at 65C....I've got to keep the temperature between 65C and 95C.

If the temp drops below 65C, open the main gas valve (MGV) until the water is greater than 95C....
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So heres what I'm thinking:

pv
-------first pass ----------------(L)---
| I
------|pv|----------------------toff------
| 3 seconds
| t4:0/dn I
-------||----------------------------( )-----
|
| T1 T1
|------|/|------T1<95---------------( )----
| T2
|------|/|------T1>65----------------(T2)---
| T1
|-----------| |-----------------------(MGV)---
| T2 |
-----| |---
|---|MGV|---------------until T2 off--------



Where its worded out I'm really not sure how to implement :-/

Thanks for the help though, this is the one I'm really needing it on :-D

First off, let's make sure we understand you ladder. Is this what you meant?

|     First Pass

|-------] [----------------(L) Igniter

|          

|     First Pass           T4:0              

|-------] [----------------(TOF) 3 Seconds

|

|    T4:0/DN

|--------] [---------------(U) Igniter

|

|      T1     Temp < 95

|------|/|-------] [-------(T1)

|

|      T2      Temp > 65

|------|/|--------] [------(T2)

|

|         T1

|-----+---] [---+----------(MGV)

      |         |

      |   T2    |

      +---] [---+

You'll have better luck if you 'draw' the ladder in notepad, then copy and paste it into your post. Put [LADDER] before the pasted text, and [/LADDER] after it. Then it will format correctly. Click 'Quote' on my post to see how I did the above ladder.



-Eric

Yep that looks a lot like it, but there has to be some action done by MGV - I'm not sure how to go about implementing that or even if the current logic is correct.

Hmm i'm looking at this, can first pass bit be used in more than 1 rung?

Also anymore help would be very appreciated :-D

stanggt200220000 said:

Yep that looks a lot like it, but there has to be some action done by MGV

Click to expand...

The Main Gas Valve is an output. The only 'action' it does is control the gas flow.

It looks like the igniter part should work with the logic you have. You need to add the logic to control the pilot valve.

The lower half of the ladder with the T1 and T2 needs work. I think I translated you ladder improperly. T1 and T2 are digital (on/off) signals, so you won't be using comparison instructions.

Based on your description, it sounds like you want the Main Gas Valve to open when the temperature drops below 65°, and REMAIN open until the temperature reaches 95°. At that point, you want it to close and REMAIN closed until the temperature drops below 65° again. Right?

I think it's important to clarify the function of the inputs:

T1 - Closed when temp < 95°; Open when temp > 95°
T2 - Closed when temp < 65°; Open when temp > 65°

Is this correct?



-Eric

stanggt200220000 said:

Hmm i'm looking at this, can first pass bit be used in more than 1 rung?

Click to expand...

You can use the first pass however often you'd like... :nodi:



-Eric

Yep that sounds exactly right, I understand the top logic part and have that worked out its the comparator part I'm not sure about

Hey eric also added in this line up top to the I latching stuff...


| First Pass

|-------] [----------------(L) PV

so now latching both I and PV....

No 'L'atches needed. It's a simple seal in circuit.

Based on Eric's description above, T1 is the 'STOP' contact, T2 is the 'START' contact, 'MGV' is both the output coil, and the seal contact around T2.