steamboiler pressure drops immidiately when superheated

[KONUK]

My company , producing motor oils, needs steam of 300 degC for some grease articles.
The new steamboiler (working pressure 4 atü) saturated steam pressure (and temperature) drops immidiately when the walfe of the superheater is opened, so we do not get any energy from the system.
Please help what we are doing not properly
Sincerely
KONUK

 

[pablo02]

We'll need some more information: what is the steam demand in lbs/hr or metric equiv.? (how much steam is needed?) What is the capacity of the new boiler? How do you start your steam system? Did you have an older boiler with no problems or is this a completely new installation? has the combustion system been tuned?

Off hand, it appears you are putting in just enough fuel to heat the boiler up to pressure, but for some reason your demand then removes the steam faster than the boiler can put heat into it... you need to narrow the cause down to why..

 

[davefitz]

It sounds like the burners are not increasing the firing rate as the pressure drops.

Normally the control logic for the burners can be simplified as follows:

Increase the firing rate to maintain the drum pressure setpoint. reduce firing rate to avoid overpressure. Trip (MFT) on lo-lo water level in the drum.

 

[yogibear1]

Konuk: Trying to decipher what your problem is. It sounds like that when you open the "valve" (walfe) on the superheater outlet, the steam pressure drops. If so, that is normal because now the steam is actually leaving the boiler, it is not bottled up anymore, and the unit is then said to be "under load". As the unit picks up load the firing rate has to increase to keep pace with the increased evaporation. If the firing rate does not pick up the pressure will eventually reach some lower point of equilbrium. Is this what is happening, if not, please furnish additional details? Thank you.

 

[KONUK]

ADDITIONAL INFO TO MY QUESTION

Superheated steam demand in lbs/hr 500
The capacity of the new boiler? 2500 lbs/hr

How do WE start OUR steam system?
Firing the burner on with full power. After the pressure is 4 bars, the superheater valve (between saturated steam part and superheater part) automatically opens. The burner still full power range. AlthOugh the last consumption department (grease department) is not using (no energy demand !)
the steam, pressure in the saturated steam part drops...

The oil factory is a new assembly, with new burner. The supplier of the burner is bankrupped and we cannot get service (it was the cheepest burner among others: "We cannot afford cheap things" prooved)....

 

[yogibear1]

I am a little surprised that such a small boiler would have a superheater; have you missed a decimal point? What is the burner oil pressure? What does the flame look like, and what about the stack is it clear or smoky?

I am curious that you say the valve opens automatically at 4 bars, or about 50 psi, while the process has no demand for the steam. The automatic valve sounds like a safety/relief valve - which would be on the drum of the boiler (according to code). I would not know of a competent boiler manufacturer that places a shut off valve between the saturated steam and superheated steam circuits. It is normal to place a shut off valve on the superheater outlet. There should also be sufficient vents and drains on the various sections of the pressure parts which are used in the warm up process to induce flow thru the circuits so they do not overheat as well as to remove excess moisture.

When warming up a boiler it is best NOT to fire the burner at full oil pressure, this could cause overheating if the (re)circulation rate in the waterwalls and superheater is not adequate.

 

[quark]

I agree with yogibear1 (I like yogibear and topcat always). The better thing is to have a control valve at the superheater outlet and control it by superheater pressure. The saturation temperature at 5 bar abs. is 150⁰C. Are you superheating it to 150⁰C further? Why can't you go for hot oil units (if you are using steam for indirect heating)?

If the boiler drum free surface volume is equal to that of super heater and it's piping system, the pressure drops as soon as the boiler outlet valve is opened(becomes zero). But as you have 2000 lbs/hr extra capacity, the pressure build up will take place later.

Regards,

 

[davefitz]

It is possible that the boiler was supplied as a saturated steam boiler and the superheater was supplied separately. A separately isolatable superheater is permitted by code but requires add'l relief valves.

If the burner control logic is not automatically adjusted to hold a fixed drum steam pressure, then the control sysetm will have to be modified to have the saturated steam outlet valve modulate to hold a set upstream drum pressure. However, it is still necessary to have a automatic override on the burners to reduce firing rate to prevent lifting saftey valves on overpressure.

 

[GenB]

Complicated... If the boiler has a stop valve to the superheater, it opens at 4 bar auto., then fills the superheater w/steam (lots of it) and press. drops at the boiler is probably normal. and it will take some time to stibilize. (for some reason the superheater was empty!),
'hope it will not meltdown, 500C is a lot of heat. anyways, it seems taht the superheater will have a steam trap to rid of condensates as well; the problem my be there. if it is a new system, the boiler may be foaming and will need to be washoff.
sounds simple but it can happen.
ER

 

[Sharik]

Wouldn't it be better to have this block valve between the boiler and superheater just open all the time. Then when the boiler starts, the boiler and superheater warm up and pressure up together. The way it is described, what is preventing the superheater from melting until there is steam flowing through it?

 

[davefitz]

Sounds like an unusual way to startup the boiler.

Most large boilers that are not provided a startup bypass system have the following limitation: during startup, the burner firing rate is limited to about 10% MCR heat imput until steam flow is established by the process of synchronizing the steam turbine.

IN the case of the small boiler described, the boiler outlet valve should be placed on inlet pressure control, set to hold the inlet pressure at 4 bar. The firing rate is limited to whatever value will limit the gas temperature to the superheater to below 550C, to avoid oxidation damage to the superheater tubes. This firing rate limit is lifted after steamflow is established thru the superheater by virtue of the boiler outlet valve opening.

To accomplish this , the following improvements are needed:

a) the boiler outlet valve must have a modulating actuator, with a control logic to hold constant 4 bar inlet pressure.This logic will force the valve to remain wide open once full firing rate is permitted. The vale would need to be a style suitable for throttling, such as a globe valve.

b) The burner must be capable of turndown to about 20% MCR firing rate.

c) the fans must be capable of turndown to about 25% MCR air flow- may require modifications to dampers, and of course assumes the air flow is controlled to ensure the exit fluegas oxygen content remains above 3% ( this implies an O2 anlayzer is installed)